doc: be explicit about who can land on staging

Refs: nodejs/node#24465

README.md

@@ -129,7 +129,8 @@ Every LTS major version has two branches in the GitHub repository: a release
 branch and a staging branch. The release branch is used to cut new releases.
 Only members of the release team should land commits into the release branch.
 The staging branch is used to land cherry-picked or backported commits from
-master that need to be included in a future release.
+master that need to be included in a future release. Only members of
+@nodejs/backporters should land commits into the staging branch.
 
 For example, for Node.js v4, there is a `v4.x` branch and a `v4.x-staging`
 branch. When commits land in master that must be cherry-picked for a future