add pull request template (filebrowser#501)

Related to filebrowser#477.

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.github/PULL_REQUEST_TEMPLATE.md

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+**Description**
+Please explain the changes you made here.
+If the feature changes current behaviour, explain why your solution is better.
+
+:rotating_light: Before submitting your PR, please read [community](https://github.com/filebrowser/community), and indicate which issues (in any of the repos) are either fixed or closed by this PR. See [GitHub Help: Closing issues using keywords](https://help.github.com/articles/closing-issues-via-commit-messages/).
+
+- [ ] DO make sure you are requesting to **pull a topic/feature/bugfix branch** (right side). Don't request your master!
+- [ ] DO make sure you are making a pull request against the **master branch** (left side). Also you should start *your branch* off *our master*.
+- [ ] DO make sure that File Browser can be successfully built. See [builds](https://github.com/filebrowser/community/blob/master/builds.md) and [development](https://github.com/filebrowser/community/blob/master/development.md).
+- [ ] DO make sure that related issues are opened in other repositories. I.e., the frontend, caddy plugins or the web page need to be updated accordingly.
+- [ ] AVOID breaking the continuous integration build.
+
+**Further comments**
+If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did, what alternatives you considered, etc.
+
+:heart: Thank you!