Doc'd how to use GitHub's "Squash and merge" button.

docs/internals/contributing/committing-code.txt

@@ -70,8 +70,11 @@ pull request "Merged in XXXXXXX" (replacing with the commit hash) after you
 merge it. Trac checks for this message format to indicate on the ticket page
 whether or not a pull request is merged.
 
-Avoid using GitHub's "Merge pull request" button on the website as it creates
-an ugly "merge commit" and makes navigating history more difficult.
+If a pull request doesn't need to be merged as multiple commits, you can use
+GitHub's "Squash and merge" button on the website. Edit the commit message as
+needed to conform to :ref:`the guidelines <committing-guidelines>` and remove
+the pull request number that's automatically appended to the message's first
+line.
 
 When rewriting the commit history of a pull request, the goal is to make
 Django's commit history as usable as possible: