docs: add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for contributing `flutter-pencilkit` package 🤗 -->
+
+## Type of change
+
+<!-- Please delete options that are not relevant. -->
+
+- [ ] Release (merge into main branch)
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+- [ ] This change requires a documentation update
+
+## What does this change?
+
+<!-- Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change. -->
+<!-- Please delete section if it is not a PR changes feature. -->
+
+Fixes #<issue_number_goes_here> 🎯
+