Add release notes for package manager

Just to keep track of where minikube is being served from besides the
places we explicitly own.  Added the brew formula as well as the arch
user repository.

AFAIK we can't do much about the Arch AUR besides flag it for update,
but we can submit pull requests for new brew formulas through the
homebrew-core repository.

RELEASING.md

@@ -79,3 +79,13 @@ gsutil cp deploy/minikube/releases.json gs://minikube/releases.json
 ```shell
 gsutil cp -r gs://minikube/releases/$RELEASE/* gs://minikube/releases/latest/
 ```
+
+## Package managers which include minikube
+
+These are downstream packages that are being maintained by others and how to upgrade them to make sure they have the latest versions
+
+Arch Linux AUR - "Flag as package out-of-date"
+https://aur.archlinux.org/packages/minikube/
+
+Brew Cask - Update Cask Formula
+https://github.com/caskroom/homebrew-cask/blob/master/Casks/minikube.rb