all: add PULL_REQUEST_TEMPLATE.md (bazel-contrib#751)

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thanks for sending a PR! Before submitting:
+
+1. If this is your first PR, please read CONTRIBUTING.md and sign the CLA
+   first. We cannot review code without a signed CLA.
+2. Please file an issue *first*. All features and most bug fixes should have
+   an associated issue with a design discussed and decided upon. Small bug
+   fixes and documentation improvements don't need issues.
+3. New features and bug fixes must have tests. Documentation may need to
+   be updated. If you're unsure what to update, send the PR, and we'll discuss
+   in review.
+-->
+
+**What type of PR is this?**
+
+> Uncomment one line below and remove others.
+>
+> Bug fix
+> Feature
+> Documentation
+> Other
+
+**What package or component does this PR mostly affect?**
+
+> For example:
+>
+> language/go
+> cmd/gazelle
+> go_repository
+> all
+
+**What does this PR do? Why is it needed?**
+
+**Which issues(s) does this PR fix?**
+
+Fixes #
+
+**Other notes for review**