Added a quick tip about PR's base branch

Author: Ruth-ikegah

content/github/collaborating-with-issues-and-pull-requests/changing-the-base-branch-of-a-pull-request.md

@@ -20,6 +20,8 @@ versions:
 4. In the base branch drop-down menu, select the base branch you'd like to [compare changes against](/github/committing-changes-to-your-project/comparing-commits#comparing-branches). ![Base branch drop-down menu ](/assets/images/help/pull_requests/pull-request-edit-base-branch.png)
 5. Read the information about changing the base branch and click **Change base**. ![Base branch change confirmation button ](/assets/images/help/pull_requests/pull-request-base-branch-confirm.png)
 
+> Quick tip: When you open a Pull Request, GitHub will set the base to the commit that branch references. If the branch is updated in the future, GitHub will not update the base branch's commit.
+
 ### Further reading
 
 - "[Creating a pull request](/articles/creating-a-pull-request)"