Add an "Updating a Go App Engine app" section to README.md.
Add an "Updating a Go App Engine app" section to README.md.

Change-Id: I887484ce788c105762edae4643854e4132fc17b1

skyeunknown084/appengine,golang/appengine,flowlo/appengine,golang/appengine,mzimmerman/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,mzimmerman/appengine,skyeunknown084/appengine,skyeunknown084/appengine,flowlo/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine