Add commit message rules to CONTRIBUTING.md

Author: Deraen

CONTRIBUTING.md

@@ -13,3 +13,14 @@ Please file bug reports and feature requests to https://github.com/metosin/compo
 * 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