Add auto-close info to PR template

Explain https://help.github.com/articles/closing-issues-via-commit-messages/

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,7 +7,9 @@
 * [ ] To the extent possible under law, I have waived all copyright and related or neighboring rights to this PR and publish it as public domain.
 -->
 
-<!-- If your PR fixes an issue, mention it here. You can also just copy the URL - GitHub will convert it for you. -->
+<!-- If your PR fixes an issue, mention it here. You can also just copy the URL - GitHub will convert it for you.
+If this PR fixes several issues, please prepend each issue url/number with the word "fix"/"fixes" or "close"/"closes" as this automatically closes the issues you metioned when the PR is merged.
+-->
 This PR fixes 
 
 ## Changes