docs(CONTRIBUTING): sync with the change of angular/angular.js#12032

Author: stevemao

CONTRIBUTING.md

@@ -157,9 +157,14 @@ format that includes a **type**, a **scope** and a **subject**:
 <footer>
 ```
 
+The **header** is mandatory and the **scope** of the header is optional.
+
 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:
 
@@ -193,6 +198,7 @@ The body should include the motivation for the change and contrast this with pre
 The footer should contain any information about **Breaking Changes** and is also the place to
 reference GitHub issues that this commit **Closes**.
 
+**Breaking Changes** should start with the word `BREAKING CHANGE:` with a space or two newlines. The rest of the commit message is then used for this.
 
 A detailed explanation can be found in this [document][commit-message-format].