Update pull_request_template.md

Author: rolandgeider

.github/pull_request_template.md

@@ -5,7 +5,8 @@
 
 ## Related Issue(s)
 
-If applicable, please link to any related issues
+If applicable, please link to any related issues (`Closes #123`,
+`Closes wger-project/other-repo#123`, `See also #123`, etc.)
 
 ## Please check that the PR fulfills these requirements