Add PR template

See plone#1174

Author: stevepiercy

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

@@ -0,0 +1,18 @@
+Thank you for your contribution to the Plone Documentation.
+
+Before submitting this pull request, please make sure you follow our guides:
+
+- [ ] [Contributing to Plone Documentation](https://6.dev-docs.plone.org/contributing/index.html)
+- [ ] [Building and Checking the Quality of Documentation](https://6.dev-docs.plone.org/contributing/setup-build.html)
+- [ ] [General Guide to Writing Documentation](https://6.dev-docs.plone.org/contributing/writing-docs-guide.html)
+
+## Issue Number
+
+- Issue #
+
+## Description
+
+_Write a description of the fixes or improvements._
+
+## Add screenshots or links to a preview of the changes
+