Add how to tick checkbox in issue template (#2851)

Many people are putting "*" instead of x, which causes the markdown to
render funny. Adding instructions to that particular section.

Backport of #2851

Author: superkhau

.github/ISSUE_TEMPLATE.md

@@ -11,6 +11,10 @@
 
 ### Bug or feature request
 
+<!--
+Put an "x" side in the item you want to check (eg. [x]).
+-->
+
 - [ ] Bug
 - [ ] Feature request