Ask for good commit practices

CONTRIBUTING.md

 ## Submitting pull requests
 
 When you issue a pull request, please specify what distribution you used for testing (if any).  Code that is committed to the master branch should work with both Debian 7 and Ubuntu 14.04 LTS.  Support for Debian 8 is coming.
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+Use good commit practices to document your changes. Don't assume the developer reviewing your commits has access to GitHub. The developer could be a future maintainer in a different environment. Similarly, as you address feedback on the pull request, do not assume the reviewer has access to GitHub.