List branch maintainers in maintainers.md

I'm not aware of this information being available
anywhere else.

doc/maintainers.md

@@ -1,5 +1,12 @@
 # Maintainers
-This page describes the current module, library, and extension maintainers of Ruby.
+This page describes the current branch, module, library, and extension maintainers of Ruby.
+
+## Branch Maintainers
+
+A branch maintainer is responsible for backporting commits into stable branches
+and publishing Ruby patch releases.
+
+[The list of current branch maintainers is available in the wiki](https://github.com/ruby/ruby/wiki/Release-Engineering).
 
 ## Module Maintainers
 A module maintainer is responsible for a certain part of Ruby.