Add pull request template (#551)

.github/pull_request_template.md

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+<!--
+  You are amazing! Thanks for contributing to our project!
+  Please, DO NOT DELETE ANY TEXT from this template! (unless instructed).
+-->
+
+## Proposed change
+<!-- 
+  Describe the big picture of your changes here to communicate to the
+  maintainers why we should accept this pull request.
+-->
+
+
+## Type of change
+<!--
+  What type of change does your pull request introduce to Home Assistant Developer Documentation? Put an `x` in the appropriate box
+  NOTE: Please, check only 1! box! 
+  If your PR requires multiple boxes to be checked, you'll most likely need to
+  split it into multiple PRs. This makes things easier and faster to code review.
+-->
+
+- [ ] Document existing features within Home Assistant
+- [ ] Document new or changing features which there is an existing pull request elsewhere
+- [ ] Spelling or grammatical corrections, or rewording for improved clarity
+- [ ] Changes to the backend of this documentation
+- [ ] Removed stale or deprecated documentation
+
+## Additional information
+<!--
+  Details are important, and help maintainers processing your PR.
+  Please be sure to fill out additional details, if applicable.
+  
+  For documentation relating to existing code please link to the relevant file on the appropriate master or dev branch (i.e.: https://github.com/home-assistant/core/blob/7c784b69638f3e2b3c91294b31a62e1058ba9709/homeassistant/components/random/sensor.py#L48-L57)
+
+  For documentation relating to new or changing code, please link to the corresponding pull request (i.e. home-assistant/core#2). This lets us easily check the status of your proposal.
+-->
+
+- This PR fixes or closes issue: fixes #
+- Link to relevant existing code or pull request: