docs(CONTRIBUTING): revert is a modifier

EG: angular@462f444

Closes angular#12032

Author: stevemao

CONTRIBUTING.md

@@ -202,6 +202,9 @@ format that includes a **type**, a **scope** and a **subject**:
 Any line of the commit message cannot be longer 100 characters! This allows the message to be easier
 to read on github as well as in various git tools.
 
+### Revert
+If the commit reverts a previous commit, it should begin with `revert: `, followed by the header of the reverted commit. In the body it should say: `This reverts commit <hash>.`, where the hash is the SHA of the commit being reverted.
+
 ### Type
 Must be one of the following: