docs: Add PR template

PULL_REQUEST_TEMPLATE.md

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+Thank you for your Pull Request!
+
+Before you submitting, please check that:
+
+- [ ] You added a brief description of the PR, e.g.:
+  - What does it do?
+  - What important points reviewers should know?
+  - Is there something left for follow-up PRs?
+- [ ] You labeled the PR with appropriate labels if you have permissions to do so.
+- [ ] You mentioned a related issue if this PR related to it, e.g. `Fixes #228` or `Related #1337`.
+- [ ] You asked any particular reviewers to review. If you aren't sure, start with GH suggestions.
+- [ ] Your PR adheres [the style guide](https://wiki.parity.io/Coding-guide)
+  - In particular, mind the maximal line length.
+  - There is no commented code checked in unless necessary.
+  - Any panickers have a proof or removed.
+- [ ] You updated any rustdocs which may have changed
+
+After you've read this notice feel free to remove it.
+Thank you!
+
+✄ -----------------------------------------------------------------------------