docs: reviewer and merge process (#4013)

In GraphQL.js working group we discussed and put process in place. But
we never officially wrote down in docs. You can watch this recording
https://youtu.be/BBxc_HmB71I?si=cvdUMI47vBsT9F2y&t=1316 or check the
transcript. These are the changes we agreed in the working group, just
putting them in writing.

---------

Co-authored-by: Benjie <benjie@jemjie.com>

.github/CONTRIBUTING.md

@@ -74,6 +74,16 @@ ensure your pull request matches the style guides, run `npm run prettier`.
 - Trailing commas,
 - Avd abbr wrds.
 
+## Review Process
+
+- Pull requests are required to pass all tests and checks before they can be merged.
+- Pull requests should be reviewed by _at least two_ members of the [`@graphql/graphql-js-reviewers`](https://github.com/orgs/graphql/teams/graphql-js-reviewers) team before they can be merged; ideally reviewers affiliated with different organizations
+- If a reviewer hasn't responded in 48 hours, feel free to ping them `@graphql/graphql-js-reviewers`.
+
+## Merge Process
+
+Once a pull request has been approved by 2 reviewers from [`@graphql/graphql-js-reviewers`](https://github.com/orgs/graphql/teams/graphql-js-reviewers) team, we must wait 2 weeks before merging the pull request, this allows for other reviewers to give feedback without rushing any merges. Pull request can be merged by a member of the [`@graphql/graphql-js-reviewers`](https://github.com/orgs/graphql/teams/graphql-js-reviewers-write) team.
+
 ## Release on NPM
 
 _Only core contributors may release to NPM._