docs(CONTRIBUTING): revert should be a valid type

EG: 462f444

Author: stevemao

CONTRIBUTING.md

@@ -215,6 +215,7 @@ Must be one of the following:
 * **test**: Adding missing tests
 * **chore**: Changes to the build process or auxiliary tools and libraries such as documentation
   generation
+* **revert**: Reverts a previous commit
 
 ### Scope
 The scope could be anything specifying place of the commit change. For example `$location`,