Adds a template for Pull Requests (flutter#28224)

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+*Replace this paragraph with a description of what this PR is doing. If you're
+modifying existing behavior, describe the existing behavior, how this PR is
+changing it, and what motivated the change. If you're changing visual
+properties, consider including before/after screenshots (and runnable code
+snippets to reproduce them).*
+
+## Related Issues
+
+*Replace this paragraph with a list of issues related to this PR from our
+[issue database]. Indicate, which of these issues are resolved or fixed by this
+PR.*
+
+## Checklist
+
+Before you create this PR confirm that it meets all requirements listed below
+by checking the relevant checkboxes (`[x]`). This will ensure a smooth and quick
+review process.
+
+- [ ] I read the [Contributor Guide] and followed the process outlined there for
+      submitting PRs.
+- [ ] My PR includes tests for *all* changed/updated/fixed behaviors (See
+      [Test Coverage]).
+- [ ] All existing and new tests are passing.
+- [ ] I updated/added relevant documentation (doc comments with `///`).
+- [ ] The analyzer (`flutter analyze --flutter-repo`) does not report any
+      problems on my PR.
+- [ ] I read and followed the [Flutter Style Guide], including
+      [Features we expect every widget to implement].
+- [ ] I signed the [CLA].
+- [ ] I am willing to follow-up on review comments in a timely manner.
+
+## Breaking Change
+
+Does your PR require Flutter developers to manually update their apps
+to accommodate your change?
+
+- [ ] Yes, this is a breaking change (Please read [Handling breaking changes]).
+- [ ] No, this is *not* a breaking change.
+
+<!-- Links -->
+[issue database]: https://github.com/flutter/flutter/issues
+[Contributor Guide]: https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
+[Test Coverage]: https://github.com/flutter/flutter/wiki/Test-coverage-for-package%3Aflutter
+[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
+[Features we expect every widget to implement]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
+[CLA]: https://cla.developers.google.com/
+[Handling breaking changes]: https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes