Add a pull request template (#930)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,20 @@
+*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*
+
+*List which issues are fixed by this PR. For larger changes, raising an issue first helps
+reduce redundant work.*
+
+## Pre-launch Checklist
+
+- [ ] I read the [Flutter Style Guide] _recently_, and have followed its advice.
+- [ ] I signed the [CLA].
+- [ ] I read the [Contributors Guide].
+- [ ] I updated/added relevant documentation (doc comments with `///`).
+- [ ] All existing and new tests are passing.
+
+If you need help, consider asking for advice on the #hackers-devrel channel on [Discord].
+
+<!-- Links -->
+[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
+[CLA]: https://cla.developers.google.com/
+[Discord]: https://github.com/flutter/flutter/wiki/Chat
+[Contributors Guide]: https://github.com/flutter/samples/blob/master/CONTRIBUTING.md