Merge pull request #7707 from issyl0/maintainers-docs-brew-mirror

docs/Maintainer-Guidelines: Document `brew mirror`

docs/Maintainer-Guidelines.md

@@ -37,6 +37,10 @@ forever. Nobody really checks if they are necessary or not. Use the
 Depend on as little stuff as possible. Disable X11 functionality if possible.
 For example, we build Wireshark, but not the heavy GUI.
 
+For [some formulae](https://github.com/Homebrew/homebrew-core/search?q=%22homebrew%2Fmirror%22&unscoped_q=%22homebrew%2Fmirror%22),
+we mirror the tarballs to our own BinTray. To do this, run
+`brew mirror <formula>` with a local checkout of a PR before it is merged.
+
 Homebrew is about Unix software. Stuff that builds to an `.app` should
 be in Homebrew Cask instead.