docs(CONTRIBUTING): add warning about forced push

Add warning about the possible consequences of a forced push

Closes angular#13747

Author: dmitriz

CONTRIBUTING.md

@@ -123,13 +123,19 @@ Before you submit your pull request consider the following guidelines:
 * If we suggest changes then:
   * Make the required updates.
   * Re-run the Angular test suite to ensure tests are still passing.
-  * Rebase your branch and force push to your GitHub repository (this will update your Pull Request):
+  * Commit your changes to your branch (e.g. `my-fix-branch`).
+  * Push the changes to your GitHub repository (this will update your Pull Request).
+
+If the PR gets too outdated we may ask you to rebase and force push to update the PR:
 
     ```shell
     git rebase master -i
     git push origin my-fix-branch -f
     ```
 
+*WARNING. Squashing or reverting commits and forced push thereafter may remove GitHub comments
+on code that were previously made by you and others in your commits.*
+
 That's it! Thank you for your contribution!
 
 #### After your pull request is merged