Add CONTRIBUTING.md (#24)

Closes #20

Author: squahtx

.github/PULL_REQUEST_TEMPLATE.md

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+### Pull Request Checklist
+
+* [ ] Pull request includes a [sign off](https://github.com/matrix-org/matrix-python-common/blob/main/CONTRIBUTING.md#sign-off)

CONTRIBUTING.md

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+# Contributing code to matrix-python-common
+
+Everyone is welcome to contribute code to matrix-python-common, provided you are
+willing to license your contributions under the same license as the project
+itself. In this case, the [Apache Software License v2](LICENSE).
+
+## Set up your development environment
+
+To contribute to matrix-python-common, ensure you have Python 3.7 and `git`
+available on your system. You'll need to clone the source code first:
+
+```shell
+git clone https://github.com/matrix-org/matrix-python-common.git
+```
+
+## Create a virtualenv
+
+To contribute to matrix-python-common, ensure you have Python 3.7 or newer and
+then run:
+
+```bash
+python3 -m venv .venv
+.venv/bin/pip install -e .[dev]
+```
+
+This creates an isolated virtual Python environment ("virtualenv") just for
+use with matrix-python-common, then installs matrix-python-common along with its
+dependencies, and lastly installs a handful of useful tools
+
+Finally, activate the virtualenv by running:
+
+```bash
+source .venv/bin/activate
+```
+
+Be sure to do this _every time_ you open a new terminal window for working on
+matrix-python-common. Activating the venv ensures that any Python commands you
+run (`pip`, `python`, etc.) use the versions inside your venv, and not your
+system Python.
+
+When you're done, you can close your terminal or run `deactivate` to disable
+the virtualenv.
+
+## Run the unit tests
+
+To make sure everything is working as expected, run the unit tests:
+
+```bash
+tox -e py
+```
+
+If you see a message like:
+
+```
+-------------------------------------------------------------------------------
+Ran 25 tests in 0.324s
+
+PASSED (successes=25)
+```
+
+Then all is well and you're ready to work!
+
+## How to contribute
+
+The preferred and easiest way to contribute changes is to fork the relevant
+project on github, and then [create a pull request](
+https://help.github.com/articles/using-pull-requests/) to ask us to pull your
+changes into our repo.
+
+Some other points to follow:
+
+ * Please base your changes on the `main` branch.
+
+ * Please follow the [code style requirements](
+   #code-style-and-continuous-integration).
+
+ * Please [sign off](#sign-off) your contribution.
+
+ * Please keep an eye on the pull request for feedback from the [continuous
+   integration system](#code-style-and-continuous-integration) and try to fix
+   any errors that come up.
+
+ * If you need to [update your PR](#updating-your-pull-request), just add new
+   commits to your branch rather than rebasing.
+
+## Code style and continuous integration
+
+matrix-python-common uses `black`, `isort` and `flake8` to enforce code style.
+Use the following script to enforce these style guides:
+
+```shell
+scripts-dev/lint.sh
+```
+
+(This also runs `mypy`, our preferred typechecker.)
+
+All of these checks are automatically run against any pull request via GitHub
+Actions. If your change breaks the build, this
+will be shown in GitHub, with links to the build results. If your build fails,
+please try to fix the errors and update your branch.
+
+## Sign off
+
+In order to have a concrete record that your contribution is intentional
+and you agree to license it under the same terms as the project's license, we've
+adopted the same lightweight approach that the Linux Kernel
+[submitting patches process](
+https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin>),
+[Docker](https://github.com/docker/docker/blob/master/CONTRIBUTING.md), and many
+other projects use: the DCO (Developer Certificate of Origin:
+https://developercertificate.org/). This is a simple declaration that you wrote
+the contribution or otherwise have the right to contribute it to Matrix:
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+If you agree to this for your contribution, then all that's needed is to
+include the line in your commit or pull request comment:
+
+```
+Signed-off-by: Your Name <your@email.example.org>
+```
+
+We accept contributions under a legally identifiable name, such as
+your name on government documentation or common-law names (names
+claimed by legitimate usage or repute). Unfortunately, we cannot
+accept anonymous contributions at this time.
+
+Git allows you to add this signoff automatically when using the `-s`
+flag to `git commit`, which uses the name and email set in your
+`user.name` and `user.email` git configs.
+
+
+## Updating your pull request
+
+If you decide to make changes to your pull request - perhaps to address issues
+raised in a review, or to fix problems highlighted by [continuous
+integration](#continuous-integration-and-testing) - just add new commits to your
+branch, and push to GitHub. The pull request will automatically be updated.
+
+Please **avoid** rebasing your branch, especially once the PR has been
+reviewed: doing so makes it very difficult for a reviewer to see what has
+changed since a previous review.
+
+## Conclusion
+
+That's it! Matrix is a very open and collaborative project as you might expect
+given our obsession with open communication. If we're going to successfully
+matrix together all the fragmented communication technologies out there we are
+reliant on contributions and collaboration from the community to do so. So
+please get involved - and we hope you have as much fun hacking on Matrix as we
+do!