Update 30_blank.md

Author: fdiblen

.github/ISSUE_TEMPLATE/30_blank.md

@@ -6,3 +6,12 @@ labels: ''
 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 you get from running `copier`.
+-->