Fix commit message guidelines (#206)

Signed-off-by: Joe Farro <joef@uber.com>

CONTRIBUTING.md

@@ -18,7 +18,14 @@ Once we've discussed your changes and you've got your code ready, make sure that
 
 * Includes tests for new functionality.
 * References the original issue in description, e.g. "Resolves #123".
-* Has a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
+* Has a [good commit message](https://chris.beams.io/posts/git-commit/):
+  * Separate subject from body with a blank line
+  * Limit the subject line to 50 characters
+  * Capitalize the subject line
+  * Do not end the subject line with a period
+  * Use the imperative mood in the subject line
+  * Wrap the body at 72 characters
+  * Use the body to explain _what_ and _why_ instead of _how_
 * Each commit must be signed by the author ([see below](#sign-your-work)).
 
 ## License