Docs: Add pull request template (photoprism#1947)

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+
+You'll be asked to accept the PhotoPrism.org Individual Contributor License Agreement https://cla-assistant.io/photoprism/photoprism once your PR will be created, please review them, your PR will be rejected if you don't accept them.
+
+Because we want to create the best possible product for our users, we have a set of guidelines which ensure that all source code contributions are acceptable:
+
+Please check the following items by replacing "[ ]" with "[x]", you can also do it when viewing the pull request once it has been created
+-->
+
+- [ ] The feature is fully implemented
+- [ ] Unit and acceptance tests have be written
+- [ ] The feature work with sqlite and mysql
+- [ ] The UI must be fully responsive
+- [ ] It has tested on multiple browsers and devices
+- [ ] Translations have been created for all languages
+- [ ] Docs have been updated
+
+<!--
+Reviewing, testing and finally merging pull requests consumes significant resources on our side. Unless it's just a small fix, it may take several months.
+
+Thanks for your patience :)
+-->
+
+<!--
+Please write below the goal of this pull request
+-->