doc: explain why we don't use the GitHub merge button
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
Docs.
Description of change
Fixes issue: https://github.com/nodejs/node/issues/8893.
Updates documentation on why the green GitHub merge button is not used. Also mentioned in the above issue was a question on why Reviewed-By
is important to add on PRs. The documentation has also been updated to answer this.