Add contribution guidelines for pull requests to copilot review

.github/copilot-instructions.md

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+A change note is required for any pull request which modifies:
+- The structure or layout of the release artifacts.
+- The evaluation performance (memory, execution time) of an existing query.
+- The results of an existing query in any circumstance.
+
+If the pull request only adds new rule queries, a change note is not required.
+Confirm that either a change note is not required or the change note is required and has been added.
+
+For PRs that add new queries or modify existing queries, also consider the following review checklist:
+- Confirm that the output format of shared queries is valid.
+- Have all the relevant rule package description files been checked in?
+- Have you verified that the metadata properties of each new query is set appropriately?
+- Do all the unit tests contain both "COMPLIANT" and "NON_COMPLIANT" cases?
+- Are all the alerts in the expected file annotated as NON_COMPLIANT in the test source file?
+- Are the alert messages properly formatted and consistent with the style guide?
+- Does the query have an appropriate level of in-query comments/documentation?
+- Does the query not reinvent features in the standard library?
+- Can the query be simplified further (not golfed!).
+
+In your review output, list only those checklist items that are not satisfied or are uncertain, but also report any other problems you find outside this checklist; do not mention checklist items that clearly pass.