Added contributing guidelines.

CONTRIBUTING.md

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+Contributing
+============
+
+Contributions are welcome!
+
+**Please carefully read this page to make the code review process go as smoothly as possible and to maximize the likelihood of your contribution being merged.**
+
+## Bug Reports
+
+For bug reports or requests [submit an issue](https://github.com/donnemartin/saws/issues).
+
+## Pull Requests
+
+The preferred way to contribute is to fork the
+[main repository](https://github.com/donnemartin/saws) on GitHub.
+
+1. Fork the [main repository](https://github.com/donnemartin/saws).  Click on the 'Fork' button near the top of the page.  This creates a copy of the code under your account on the GitHub server.
+
+2. Clone this copy to your local disk:
+
+        $ git clone git@github.com:YourLogin/saws.git
+        $ cd saws
+
+3. Create a branch to hold your changes and start making changes. Don't work in the `master` branch!
+
+        $ git checkout -b my-feature
+
+4. Work on this copy on your computer using Git to do the version control. When you're done editing, run the following to record your changes in Git:
+
+        $ git add modified_files
+        $ git commit
+
+5. Push your changes to GitHub with:
+
+        $ git push -u origin my-feature
+
+6. Finally, go to the web page of your fork of the SAWS repo and click 'Pull Request' to send your changes for review.
+
+### GitHub Pull Requests Docs
+
+If you are not familiar with pull requests, review the [pull request docs](https://help.github.com/articles/using-pull-requests/).
+
+### Code Quality
+
+Ensure your pull request
+
+* Is covered by [unit tests](#unit-tests-and-code-coverage)
+* Passes [continuous integration](#continuous-integration)
+* Is covered by [documentation](#documentation)
+
+Review the following [style guide](https://google-styleguide.googlecode.com/svn/trunk/pyguide.html).
+
+Run code checks and fix any issues:
+
+    $ cd utility_scripts
+    $ ./run_code_checks.sh
+
+### Installation
+
+Refer to the [Installation](https://github.com/donnemartin/saws#installation) and [Developer Installation](https://github.com/donnemartin/saws#developer-installation) sections.