Update 10_generated_package.md

Author: fdiblen

.github/ISSUE_TEMPLATE/10_generated_package.md

@@ -6,6 +6,12 @@ labels: generated-package
 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
 <!--
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