Signal-Android/.github/PULL_REQUEST_TEMPLATE.md

<!-- You can remove this first section if you have contributed before -->
### First time contributor checklist
<!-- replace the empty checkboxes [ ] below with checked ones [x] accordingly -->
- [ ] I have read [how to contribute](https://github.com/signalapp/Signal-Android/blob/master/CONTRIBUTING.md) to this project
- [ ] I have signed the [Contributor License Agreement](https://whispersystems.org/cla/)

### Contributor checklist
<!-- replace the empty checkboxes [ ] below with checked ones [x] accordingly -->
- [ ] I am following the [Code Style Guidelines](https://github.com/signalapp/Signal-Android/wiki/Code-Style-Guidelines)
- [ ] I have tested my contribution on these devices:
 * Device A, Android X.Y.Z
 * Device B, Android Z.Y
 * Virtual device W, Android Y.Y.Z
- [ ] My contribution is fully baked and ready to be merged as is
- [ ] I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the `Fixes #1234` [syntax](https://help.github.com/articles/closing-issues-via-commit-messages/)

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### Description
<!--
Describe briefly what your pull request proposes to fix. Especially if you have more than one commit, it is helpful to give a summary of what your contribution as a whole is trying to solve.
Also, please describe shortly how you tested that your fix actually works.
-->