Update contributing guide

Added this text to the contributing guide:

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Commit messages should be descriptive, clear, and meaningful. Every logical change should be a separate commit, while avoiding excessive commits. While it's not necessary to [squash](http://davidwalsh.name/squash-commits-git) Pull Requests to a single commit, Pull Requests should contain the minimum number of commits to clearly express the changes made. If an excessive number of commits are included in a Pull Request, the request may be closed and the submitter asked to re-submit after squashing the commits.