No issue: Adds screenshots to the PR checklist (#5089)

.github/PULL_REQUEST_TEMPLATE

@@ -4,6 +4,7 @@
 <!-- Before submitting the PR, please address each item -->
 - [ ] **Quality**: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
 - [ ] **Tests**: This PR includes thorough tests or an explanation of why it does not
+- [ ] **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
 
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