adding contributing guidelines

CONTRIBUTING.md

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+# How to contribute
+
+Contributions to Magit are highly welcome. Here are a few guidelines that will
+help your patches hit upstream as soon as possible.
+
+## Branching scheme
+
+Magit uses 3 main branches for its lifecyle: `maint`, `master` and `next`.
+
+* `maint` contains the set of patches that will definitely make it into the next
+  minor release.
+* `master` contains the set of patches that will highly likely make it into the
+  next major release.
+* `next` contains patches that necessitate some additional checks/improvements
+  before they're integrating into a release.
+
+## Making changes
+
+A good practice is to create a topic branch off Magit, from the branch you
+target for *final* inclusion. This should *always* be either `maint` or
+`master`.
+
+If you branch off `next`, you'll only put more overhead on the maintainer's
+shoulders, and the integration will suffer from additional delays.
+
+Please make sure your commits are well-organized and "atomic" (hitting a single
+well-defined target each).
+
+Please also make sure you check that byte-compilation completes without errors
+of warnings, and that tests run without failures.
+
+Bonus points if you add tests to cover the feature you're hacking.
+
+## Submitting changes
+
+The preferred way of submitting your changes is to open a pull request on the
+central Magit GitHub repository (https://github.com/magit/magit).
+
+Alternately, you can send your patches to the Magit mailing list
+(magit@googlegroups.com), using `git send-email`.