Update 20_template.md

Author: fdiblen

.github/ISSUE_TEMPLATE/20_template.md

@@ -6,6 +6,12 @@ labels: template
 assignees: ''
 
 ---
+
+## Checklist before submitting an issue
+
+- [ ] I have checked the [existing issues](https://github.com/NLeSC/python-template/issues) and couldn't find an issue about this bug.
+
+## Issue details
 <!--
-Whatever you write here must have something to do with the code of the template itself, i.e. before you run `copier`.
+Whatever you write here must have something to do with the code you get from running `copier`.
 -->