Add contributing guidelines to OneBusAway Android

After adding this file to the project on Github, Github will automatically include a header over the issue editor
saying "Please review the guidelines for contributing to this repository", with "guidelines for contributing" being hyperlinked to this markdown file.  See https://github.com/blog/1184-contributing-guidelines for details.

@paulcwatts This is pretty close to a copy-and-paste from the OBA iPhone project, with various items changed to reflect Android instead of iPhone.  Please feel free to edit further to match your expectations for the project.  My main motivation for submitting this is the implicit license agreement at the bottom, to help keep the code cleanly licensed under Apache 2.0.