Expand master branch change section:

- Add item to list
- Explain how to remove the master branch

Author: HonkingGoose

book/03-git-branching/sections/branch-management.asc

@@ -175,3 +175,10 @@ Now you have a few more tasks in front of you to complete the transition:
 * Release scripts.
 * Repository configuration settings (like the default branch that is targeted for pull-requests, branch-protection measures, etc).
 * References to the old branch in documentation.
+* Close/merge pull requests that target the old branch
+
+After you've done all these tasks, and are certain the main branch performs just as the _master_ branch, you can delete the _master_ branch:
+[source, console]
+----
+$ git push origin --delete master
+----