add horizontal rule to GitHub pull request template

git-svn-id: http://svn.redmine.org/redmine/trunk@16270 e93f8b46-1217-0410-a6f0-8f06a7374b81

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,6 @@
 
+____________________________________________________________________
+
 **Do not send a pull request to this GitHub repository**.
 
 For more detail, please see [official website] wiki [Contribute].