Add contribution instructions.

CONTRIBUTING.md

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+Drush is built by people like you! Please [join us](https://github.com/drush-ops/drush).
+
+## Git and Pull requests
+* Contributions are submitted, reviewed, and accepted using Github pull requests. [Read this article](https://help.github.com/articles/using-pull-requests) for some details. We use the _Fork and Pull_ model, as described there.
+* The latest changes are in your `master` branch.
+* Make a new branch for every feature you're working on.
+* Try to make clean commits that are easy readable (including descriptive commit messages!)
+* Test before you push. get familiar with Unish, our test suite. Docs are at /tests/README.md
+* Make small pull requests that are easy to review but make sure they do add value by themselves.
+* We maintain branches named 6.x, 5.x, etc. These are release branches. From these branches, we make new tags for patch and minor versions.
+
+## Coding style
+* Do write comments. You don't have to comment every line, but if you come up with something thats a bit complex/weird, just leave a comment. Bear in mind that you will probably leave the project at some point and that other people will read your code. Undocumented huge amounts of code are nearly worthless!
+* We use [Drupal's coding standards](https://drupal.org/coding-standards).
+* Don't overengineer. Don't try to solve any possible problem in one step, but try to solve problems as easy as possible and improve the solution over time!
+* Do generalize sooner or later! (if an old solution, quickly hacked together, poses more problems than it solves today, refactor it!)
+* Keep it compatible. Do not introduce changes to the public API, or configurations too lightly. Don't make incompatible changes without good reasons!
+
+## Documentation
+* The docs are in the `docs/` and `examples/` folders in the git repository, so people can easily find the suitable docs for the current git revision. You can read these from within Drush, with the `drush topic` command.
+* Documentation should be kept up-to-date. This means, whenever you add a new API method, add a new hook or change the database model, pack the relevant changes to the docs in the same pull request.
+* You can build the docs e.g. produce html, using `drush help --html`. This page also resides at http://www.drush.org. You might enjoy prettier and more comprehensive version at [Drush Commands](http://www.drushcommands.com)