Update README.md to be clearer about the purpose of this repository.
Update README.md to be clearer about the purpose of this repository.

This is for both classic App Engine and Managed VMs.

Change-Id: Iaa2ee15622ac276af9a36707c465d99808870cca

golang/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,golang/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,mzimmerman/appengine