Adds the PR template from weave-gitops to this repo

.github/pull_request_template.md

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+<!--
+Use # to add the issue this pull request is related to.
+nb: This is the Github issue number, not a Zenhub link.
+Do not use any punctuation or bullet points.
+eg:
+Closes #1234
+Fixes #5678
+-->
+Closes
+
+<!-- Describe what has changed in this PR -->
+**What changed?**
+
+
+<!-- Tell your future self why have you made these changes -->
+**Why was this change made?**
+
+
+<!--
+Explain to your reviewers the key implementation points, including why you made
+certain choices in favour of others. Highlight key areas of the code which need
+extra attention, and also indicate which parts are less relevant (eg renaming,
+refactoring, etc
+-->
+**How was this change implemented?**
+
+
+<!--
+How have you verified this change/product value? Tested locally?
+Added integration/acceptance test(s)?
+Unit tests are required.
+-->
+**How did you validate the change?**
+
+
+<!--
+Is it notable for release? e.g. schema updates, configuration or data migration
+required? If so, please mention it.
+-->
+**Release notes**
+
+
+<!-- Are there any documentation updates that should be made for these changes? -->
+**Documentation Changes**