Add guide for contributing to project

CONTRIBUTING.md

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+## Contributing to Exchange Web Services Java API
+
+There are several ways to contribute to the project:
+
+* Report bugs and features in the [issue tracker](https://github.com/officedev/ews-java-api/issues).
+* Submit and review pull requests
+* Help with documentation
+* Help with testing
+
+GitHub supports [markdown](http://github.github.com/github-flavored-markdown/), so when filing bugs make sure you check the formatting before clicking submit.
+
+### Contributing code and content
+Before submitting a feature or substantial code contribution please discuss it with the team and ensure it follows the product roadmap.  You might also read these two blogs posts on contributing code:
+
+* [Open Source Contribution Etiquette](http://tirania.org/blog/archive/2010/Dec-31.html) by Miguel de Icaza
+* [Don't "Push" Your Pull Requests](http://www.igvita.com/2011/12/19/dont-push-your-pull-requests/) by Ilya Grigorik.
+
+### Pull Requests
+If you don't know what a pull request is read the "[Using pull requests](https://help.github.com/articles/using-pull-requests)" article.
+
+Some guidelines for pull requests:
+
+* Use a descriptive title and description.
+* Include a single logical change.
+* Base on master branch - once accepted, can be ported to stable branches.
+* Should cleanly merge with target branch.
+
+### Commit Messages
+1. Separate subject from body with a blank line
+2. Limit the subject line to 50 characters
+3. Capitalize the subject line
+4. Do not end the subject line with a period
+5. Use the imperative mood in the subject line (e.g. Fix #123: Make pigs fly).
+6. Wrap the body at 72 characters
+7. Use the body to explain what and why.  The how should be mostly covered by the diff.
+
+References:
+
+* [How to Write a Git Commit Message](http://chris.beams.io/posts/git-commit/)
+* [A Note About Git Commit Messages](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)
+* [Guidelines for Commit Messages](https://wiki.gnome.org/Git/CommitMessages)
+* [On commit messages](http://who-t.blogspot.de/2009/12/on-commit-messages.html)