Add CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+
+Contributions are welcome.
+
+Please file bug reports and feature requests to https://github.com/metosin/testit/issues.
+
+## Making changes
+
+* Fork the repository on Github
+* Create a topic branch from where you want to base your work (usually the master branch)
+* Check the formatting rules from existing code (no trailing whitepace, mostly default indentation)
+* Ensure any new code is well-tested, and if possible, any issue fixed is covered by one or more new tests
+* Verify that all tests pass using ```lein midje```
+* Push your code to your fork of the repository
+* Make a Pull Request
+
+## 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
+    - "Add x", "Fix y", "Support z", "Remove x"
+6. Wrap the body at 72 characters
+7. Use the body to explain what and why vs. how
+
+For comprehensive explanation read this [post by Chris Beams](http://chris.beams.io/posts/git-commit/#seven-rules).