Add Contribution File

I mainly wanted to add this file so that people adding new sites would add test usernames.  But, this document should be filled out with more detail on the other types of changes....

CONTRIBUTING.md

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+# How To Contribute To Sherlock
+First off, thank you for the help!
+
+There are many ways to contribute.  Here is some high level grouping.
+
+## Adding New Sites
+
+Please look at the Wiki entry on
+[adding new sites](https://github.com/TheYahya/sherlock/wiki/Adding-Sites-To-Sherlock)
+to understand the issues.
+
+Any new sites that are added need to have a username that has been claimed, and one
+that is unclaimed documented in the site data.  This allows the regression tests
+to ensure that everything is working.
+
+## Adding New Functionality
+
+Please ensure that the content on your branch passes all tests before submitting a pull request.