Add PR template to "help" the contributor and the reviewers

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
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+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+## Summary
+
+<!-- Describe what this PR does and why. -->
+
+## Related Issues
+
+<!-- Link any related GitHub issues: Closes #123, Fixes #456 -->
+
+## Checklist
+
+### Specification
+- [ ] Spec changes are included in `core-spec/` and follow the existing structure
+- [ ] Spec changes have been discussed on the mailing list or in a linked issue
+- [ ] Breaking changes to the spec are clearly called out in the summary
+
+### Ontology
+- [ ] Ontology changes in `ontology/` are consistent with spec changes
+- [ ] New or modified terms are defined and documented
+
+### Converters
+- [ ] Converter logic in `converters/` is updated to reflect spec or ontology changes
+- [ ] New converters include tests under the converter's test directory
+
+### Validation
+- [ ] Validation rules in `validation/` are updated if the spec changed
+- [ ] New validation cases are covered by tests
+
+### Documentation
+- [ ] `docs/` is updated to reflect any user-facing changes
+- [ ] New features or behaviors are documented with examples where appropriate
+- [ ] `CONTRIBUTING.md` is updated if the contribution process changed
+
+### Examples
+- [ ] `examples/` are added or updated for any new spec constructs or converter support
+
+### Tests
+- [ ] All existing tests pass (`pytest` / CI green)
+- [ ] New functionality is covered by tests
+
+### Compliance
+- [ ] ASF license headers are present on all new source files
+- [ ] No third-party dependencies are added without PMC/IPMC approval