Adds Contributor guidelines for developers

Takes the guidelines already published on ThinkUp Documentation
and boils them down for a quicker read.

Links back for those who want to read up fully on the subject.

References:

- http://thinkupapp.com/docs/contribute/developers/devfromsource.html
- http://thinkupapp.com/docs/contribute/developers/pullrequestchecklist.html

Closes #1384

CONTRIBUTING.md

@@ -0,0 +1,85 @@
+Thanks for wanting to contribute!
+
+Below is a quick run down on the workflow, dos & don'ts, and a checklist for developers
+wanting to contribute to ThinkUp.
+
+## Contributor workflow
+
+This is a quick summary, but you can read the full version at
+[Develop from Source - ThinkUp Documentation](http://thinkupapp.com/docs/contribute/developers/devfromsource.html)
+
+1. Fork the [project from GitHub](https://github.com/ginatrapani/ThinkUp/)
+
+2. Clone the fork to your server
+
+        $ git clone git@github.com:username/ThinkUp.git
+
+3. Set up a remote upstream so that you can keep up-to-date with the source repo changes
+
+        $ git remote add upstream git://github.com/ginatrapani/ThinkUp.git
+
+    You can verify you've correctly set up the remote by running `$ git remote -v` to
+    list the names and their URIs.
+
+4. Create a specific feature branch to develop on
+
+        $ git checkout -b ###-description-of-feature-or-bugfix
+
+    `###` will refer to an open issue ticket on GitHub.
+
+5. Edit and test the changes on your development server
+
+6. Rebase (and fix any conflicts) against the upstream master branch
+
+        $ git fetch upstream
+        $ git checkout master
+        $ git rebase upstream/master
+        $ git checkout ###-description
+        [make sure all is committed as necessary in branch]
+        $ git rebase master
+
+    Get in the habit of continually rebasing your fork against the upstream.
+
+7. Squash all X commits that pertain to the issue into one clean, descriptive commit
+
+        $ git rebase -i HEAD-X
+
+8. Push the release candidate branch to GitHub
+
+        $ git push origin ###-description-rc
+
+9. Issue pull request on GitHub.
+
+
+## Pull request checklist
+
+To increase the chances your contribution will be accepted into the master branch, make sure:
+
+- Changes adhere to the ThinkUp coding standards - Check the
+[Code Style Guide](http://thinkupapp.com/docs/contribute/developers/writecode/styleguide/index.html)
+on what it should look like.
+
+- Code is thoroughly documented - Make sure your classes and methods follow PHPDoc standards.
+
+- All existing unit tests pass - Any code that makes existing tests in `/tests` fail will not be
+merged. Read the [tests README](http://github.com/ginatrapani/thinkup/blob/master/tests/README.md)
+for more on how to set up, run, and write tests.
+
+- Add regression tests for new code - If you've fixed a bug, add a test that fails in the
+current development tree, but passes due to your fix. If it's a new feature, object method
+or plugin, make sure you've also added thorough and complete tests that demonstrate that
+it works.
+
+- Rebase your work against the current state of the development tree - Help keep the
+ThinkUp commit trees clean.
+
+
+## Dos and don'ts
+
+- Don't develop on the master branch - Always create a development branch specific to
+the issue
+- Don't merge the upstream master with your development branch - Rebase your branch on top
+of master
+- Do create branches for each issue you are working on
+- Do squash multiple commits for a clean commit history and before pushing
+- Do keep .gitignore clean - Don't add test files that are specific to your setup.