docs(CONTRIBUTING): how to write a breaking change

Closes angular#12032

Author: stevemao

CONTRIBUTING.md

@@ -235,6 +235,7 @@ 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].