chore: mention caretaker in code_reviews doc (#5169)

Author: jelbourn

CODE_REVIEWS.md

@@ -14,4 +14,19 @@
   * 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. 
+4. Once the LGTM label is applied, either the author or the reviewer can add the "merge-ready"
+   label to indicate that the PR is ready to be merged.
+5. The party responsible for merging PRs will do so.
+
+## How PRs are merged
+The team has a weekly rotation for the "caretaker" who is responsible for merging PRs. Before being
+merged, the caretaker runs PRs through Google's internal presubmit system. This process helps
+greatly in keeping the library stable by running against the tests of many applications inside of
+Google. Due to the volume of tests involved, this process means that there can be some delay
+between a PR being approved and it being merged.
+
+The "merge safe" label means that the change doesn't affect the library itself (or the demo-app),
+and thus can be merged without this extra presubmit.
+
+The "presubmit failure" label means that the PR has encountered some failure during presubmit and
+needs further investigation by the team.