Add a few GitHub-specific files

.github/FUNDING.yml

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+# These are supported funding model platforms
+
+github: bbatsov
+patreon: bbatsov
+ko_fi: bbatsov
+liberapay: bbatsov
+custom: https://www.paypal.me/bbatsov

.github/ISSUE_TEMPLATE.md

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+*Use the template below when reporting bugs. Please, make sure that
+you're running the latest stable Projectile and that the problem you're reporting
+hasn't been reported (and potentially fixed) already.*
+
+**Remove all of the placeholder text in your final report!**
+
+## Expected behavior
+
+## Actual behavior
+
+## Steps to reproduce the problem
+
+*This is extremely important! Providing us with a reliable way to reproduce
+a problem will expedite its solution.*
+
+## Environment & Version information
+
+### super-save version
+
+*E.g. 0.4*
+
+### Emacs version
+
+*E.g. 24.5* (use <kbd>C-h C-a</kbd> to see it)
+
+### Operating system
+
+*E.g. Windows 10*

.github/PULL_REQUEST_TEMPLATE.md

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+**Replace this placeholder text with a summary of the changes in your PR.
+The more detailed you are, the better.**
+
+-----------------
+
+Before submitting a PR make sure the following things have been done (and denote this
+by checking the relevant checkboxes):
+
+- [ ] The commits are consistent with our [contribution guidelines](../blob/master/CONTRIBUTING.md)
+- [ ] You've added tests (if possible) to cover your change(s)
+- [ ] All tests are passing ([`eldev test`](https://github.com/doublep/eldev))
+- [ ] The new code is not generating bytecode or `M-x checkdoc` warnings
+- [ ] You've updated the [changelog](../blob/master/CHANGELOG.md) (if adding/changing user-visible functionality)
+- [ ] You've updated the readme (if adding/changing user-visible functionality)
+
+Thanks!

.github/workflows/test.yml

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+name: CI
+
+on:
+  push:
+    paths-ignore: ['**.md', '**.adoc']
+  pull_request:
+    paths-ignore: ['**.md', '**.adoc']
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    continue-on-error: ${{matrix.emacs_version == 'snapshot'}}
+
+    strategy:
+      matrix:
+        # Earliest supported + latest in each stable branch + snapshot.
+        emacs_version: ['25.1', '25.3', '26.3', '27.1', '28.1', 'snapshot']
+
+    steps:
+    - name: Set up Emacs
+      uses: purcell/setup-emacs@master
+      with:
+        version: ${{matrix.emacs_version}}
+
+    - name: Install Eldev
+      run: curl -fsSL https://raw.github.com/doublep/eldev/master/webinstall/github-eldev | sh
+
+    - name: Check out the source code
+      uses: actions/checkout@v2
+
+    - name: Lint the project
+      run: |
+        eldev -dtT -C compile --warnings-as-errors