Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+### Changelog entry <!-- a user-readable short description of changes introduced in this PR -->
+
+...
+
+### Changelog category <!-- remove all except one -->
+
+* New feature
+* Experimental feature
+* Improvement
+* Performance improvement
+* Bugfix 
+* Backward incompatible change
+* Documentation (changelog entry is not required)
+* Not for changelog (changelog entry is not required)
+
+### Additional information
+
+...