Create PULL_REQUEST_TEMPLATE.md (#1904)

.github/PULL_REQUEST_TEMPLATE.md

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+##### Instructions
+
+- Look for and complete the section relevant to your submission. Delete the others, including these `Instructions`.
+- `{{cask_file}}` is used a placeholder to the actual cask you’re submitting/editing (if applicable).
+- If there’s a checkbox you can’t complete for any reason, that’s OK. Just explain in detail why you weren’t able to do so.
+
+### Changes to a cask
+#### Editing an existing cask
+
+- [ ] Commit message includes cask’s name (and new version, if applicable).
+- [ ] `brew cask audit --download {{cask_file}}` is error-free.
+- [ ] `brew cask style --fix {{cask_file}}` left no offenses.
+
+#### Adding a new cask
+
+- [ ] Checked there aren’t open [pull requests](https://github.com/caskroom/homebrew-versions/pulls) for the same cask.
+- [ ] Checked there aren’t closed [issues](https://github.com/caskroom/homebrew-versions/issues) where that cask was already refused.
+- [ ] Checked the cask follows the requirements of [acceptable casks](https://github.com/caskroom/homebrew-versions#acceptable-casks).
+- [ ] When naming the cask, followed the [token reference](https://github.com/caskroom/homebrew-cask/blob/master/doc/cask_language_reference/token_reference.md).
+- [ ] Commit message includes cask’s name.
+- [ ] `brew cask audit --download {{cask_file}}` is error-free.
+- [ ] `brew cask style --fix {{cask_file}}` left no offenses.
+- [ ] `brew cask install {{cask_file}}` worked successfully.
+- [ ] `brew cask uninstall {{cask_file}}` worked successfully.