Added PR template.

.github/PULL_REQUEST_TEMPLATE.md

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+This template is not intended to be prescriptive, but to help us review pull requests it would be useful if you included as much of the following information as possible:
+
+- What does the pull request do?
+- What is the current behavior?
+- What is the updated/expected behavior with this PR?
+- How was the solution implemented (if it's not obvious)?
+
+Checklist:
+
+- [ ] Added unit tests (if possible)?
+- [ ] Added XML documentation to any related classes?
+- [ ] Consider submitting a PR to https://github.com/AvaloniaUI/Avaloniaui.net with user documentation
+
+If the pull request fixes issue(s) list them like this:
+
+Fixes #123
+Fixes #456