github: Add PR template

Gives structure to pull requests and make it easier for us to review.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+<!-- Please read our contributing guide before opening a PR: https://github.com/polybar/polybar/blob/master/CONTRIBUTING.md -->
+
+## What type of PR is this? (check all applicable)
+
+* [ ] Refactor
+* [ ] Feature
+* [ ] Bug Fix
+* [ ] Optimization
+* [ ] Documentation Update
+* [ ] Other: *Replace this with a description of the type of this PR*
+
+## Description
+<!--
+  Document user-facing changes in this PR (for example: new config options, changed behavior).
+
+  You can also motivate design decisions here.
+-->
+
+## Related Issues & Documents
+<!-- For example: Fixes #1234, Closes #6789 -->
+
+## Documentation (check all applicable)
+
+* [ ] This PR requires changes to the Wiki documentation (describe the changes)
+* [ ] This PR requires changes to the documentation inside the git repo (please add them to the PR).
+* [ ] Does not require documentation changes