Create PULL_REQUEST_TEMPLATE.md

Author: gaearon

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+
+Starting a new translation?
+
+See the instructions here: https://github.com/reactjs/translations.react.dev#starting-a-new-translation
+
+Make sure that you:
+
+* Describe your existing translation experience.
+* Find at least one other maintainer and ask them to comment on this PR.
+
+Thanks!
+
+-->