added contribution guidelines

CONTRIBUTING.md

@@ -0,0 +1,31 @@
+# Contributing
+
+If you discover issues, have ideas for improvements or new features, or
+want to contribute a new module, please report them to the
+[issue tracker][1] of the repository or submit a pull request. Please,
+try to follow these guidelines when you do so.
+
+## Issue reporting
+
+* Check that the issue has not already been reported.
+* Check that the issue has not already been fixed in the latest code
+  (a.k.a. `master`).
+* Be clear, concise and precise in your description of the problem.
+* Open an issue with a descriptive title and a summary in grammatically correct,
+  complete sentences.
+* Include any relevant code to the issue summary.
+
+## Pull requests
+
+* Read [how to properly contribute to open source projects on Github][2].
+* Use a topic branch to easily amend a pull request later, if necessary.
+* Write [good commit messages][3].
+* Use the same coding conventions as the rest of the project.
+* Verify your Emacs Lisp code with `checkdoc` (<kbd>C-c ? d</kbd>).
+* Open a [pull request][4] that relates to *only* one subject with a clear title
+  and description in grammatically correct, complete sentences.
+
+[1]: https://github.com/bbatsov/prelude/issues
+[2]: http://gun.io/blog/how-to-github-fork-branch-and-pull-request
+[3]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+[4]: https://help.github.com/articles/using-pull-requests