Add basic contribution guidelines.

Author: timss

.github/CONTRIBUTING.md

@@ -0,0 +1,31 @@
+Contributing
+============
+
+Thank you for contributing to vimperator-labs! 
+
+Following are some guidelines designed to make the process as smooth as
+possible. If you have any questions, feel free to drop by
+`#vimperator@freenode`.
+
+Issues
+======
+
+A few things to keep in mind when creating a new issue:
+
+- [ ] Include OS/distro, Firefox/Thunderbird and Vimperator/Muttator version
+- [ ] Attach relevant configuration or RC file if applicable (e.g.
+  `.vimperatorrc`)
+- [ ] Explain steps to reproduce
+- [ ] Describe what you have tried
+- [ ] Check if a fresh profile solves the issue
+    - `$ firefox -no-remote -P`
+- [ ] Confirm if it works without your configuration
+    - `$ firefox -no-remote -P <fresh profile> -vimperator "+u NONE"`
+
+Pull requests
+=============
+
+- [ ] Title and commit message(s) should include relevant issue ID(s)
+- [ ] For any new or changed feature, asciidoc documentation and an entry in
+  the NEWS file is required for the patch to be accepted. 
+