Added GitHub PR template.

.github/PULL_REQUEST_TEMPLATE.md

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+Thank you for your interest into contributing to OctoPrint-PSUControl, it's highly appreciated!
+
+Before submitting please make sure you have ticked all points on this checklist:
+
+  * [ ] Your PR targets the devel branch.
+  * [ ] Your PR was opened from a custom branch on your repository (no PRs from your version of master or devel please)
+  * [ ] Your PR only contains relevant changes: no unrelated files, no dead code, ideally only one commit - rebase your PR if necessary!
+  * [ ] Your changes follow the existing coding style.
+  * [ ] You have tested your changes (please state how!)
+
+Feel free to delete all this help text, then describe your PR further. You may use the template provided below to do that. The more details the better!
+
+---
+
+#### What does this PR do and why is it necessary?
+
+#### How was it tested? How can it be tested by the reviewer?
+
+#### Any background context you want to provide?
+
+#### What are the relevant tickets if any?
+
+#### Screenshots (if appropriate)
+
+#### Further notes
+