chore(): add guide for code reviews

CODE_REVIEWS.md

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+# Code reviews for Angular Material
+
+* Before any coding begins on new, large, or breaking work, a design discussion should take place.
+* All code changes require a review and approval.
+* All behaviors should be covered by unit tests in the same PR.
+* Large changes should be accompanied by corresponding e2e tests in the same PR. 
+* Authors should attempt to keep PRs to 200 - 300 line changes.
+
+## Workflow
+1. The code author sends a PR for review. This request should include:
+  * A mention of the intended reviewer(s) (e.g., `@jelbourn`)
+  * A high-level description of the change being made.
+  * Links to any relevant issues.
+  * Screenshots (for visual changes or new additions)
+2. Reviews provide comments and the author responds / makes changes. Repeat until LGTM.
+3. One or more of the reviewers applies the "LGTM" label.
+4. The party responsible for merging PRs will do so.