No issue: Add AFTER-MERGE checklist item for PR

.github/PULL_REQUEST_TEMPLATE

@@ -7,6 +7,9 @@
 - [ ] **Screenshots**: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
 - [ ] **Accessibility**: The code in this PR follows [accessibility best practices](https://github.com/mozilla-mobile/shared-docs/blob/master/android/accessibility_guide.md) or does not include any user facing features
 
+### After merge
+- [ ] **Milestone**: Make sure issues finished by this pull request are added to the [milestone](https://github.com/mozilla-mobile/fenix/milestones) of the version currently in development.
+
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