Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributions
+
+Contributions and pull requests are welcome.  Please take note of the following guidelines:
+
+* **Work and push on `next` branch**. `git remote update; git checkout origin/next` before hacking. Exceptions for bugs in `master` branch and simple typo fix.
+* Always rebase your work before push: `git pull --rebase origin next`
+* Adhere to the existing style as much as possible; notably, tabulation indents and long-form keywords. Try to add comments for important changes/addings.
+* Make targeted pull request: propose one feature/bugfix, and push only commit(s) related to this feature/bugfix.
+* Any push to your pull request will trigger a regression job on [travis](https://travis-ci.org/jreybert/vimagit).
+
+# Bugs
+
+* To make it easier to reproduce, please supply the following:
+  * the version of `vim` or `nvim`
+  * the version of `git`
+  * the SHA1 of vimagit you're using
+  * the OS that you're using, including terminal emulator, GUI vs non-GUI
+  * a step by step command to reproduce the bug
+