Add documentation for potential contributors

Author: allisonurban

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+Pull Requests
+-------------
+Here are some reasons why a pull request may not be merged:
+
+1. It hasn’t been reviewed.
+2. It doesn’t include specs for new functionality.
+3. It doesn’t include documentation for new functionality.
+4. It changes behavior without changing the relevant documentation, comments, or specs.
+5. It changes behavior of an existing public API, breaking backward compatibility.
+6. It breaks the tests on a supported platform.
+7. It doesn’t merge cleanly (requiring Git rebasing and conflict resolution).
+
+If you would like to help in this process, you can start by evaluating open pull requests against the criteria above. For example, if a pull request does not include specs for new functionality, you can add a comment like: “If you would like this feature to be added to Thor, please add specs to ensure that it does not break in the future.” This will help move a pull request closer to being merged.  
+
+Include this emoji in the top of your ticket to signal to us that you read this file: 🌈