[MRG] Add the meaning of MRG and MRG+1 in the PR in docs. (#8406)

CONTRIBUTING.md

@@ -79,7 +79,9 @@ following rules before you submit a pull request:
 
 -  Please prefix the title of your pull request with `[MRG]` (Ready for
    Merge), if the contribution is complete and ready for a detailed review.
-   An incomplete contribution -- where you expect to do more work before
+   Two core developers will review your code and change the prefix of the pull
+   request to `[MRG + 1]` and `[MRG + 2]` on approval, making it eligible
+   for merging. An incomplete contribution -- where you expect to do more work before
    receiving a full review -- should be prefixed `[WIP]` (to indicate a work
    in progress) and changed to `[MRG]` when it matures. WIPs may be useful
    to: indicate you are working on something to avoid duplicated work,