change README for app-engine example because of goapp deprecation (#1324)

the App Engine SDK is superseded by the Cloud SDK

examples/app-engine/README.md

@@ -1,7 +1,8 @@
 # Guide to run Gin under App Engine LOCAL Development Server
 
 1. Download, install and setup Go in your computer. (That includes setting your `$GOPATH`.)
-2. Download SDK for your platform from here: `https://developers.google.com/appengine/downloads?hl=es#Google_App_Engine_SDK_for_Go`
+2. Download SDK for your platform from [here](https://cloud.google.com/appengine/docs/standard/go/download): `https://cloud.google.com/appengine/docs/standard/go/download`
 3. Download Gin source code using: `$ go get github.com/gin-gonic/gin`
-4. Navigate to examples folder: `$ cd $GOPATH/src/github.com/gin-gonic/gin/examples/`
-5. Run it: `$ goapp serve app-engine/`
+4. Navigate to examples folder: `$ cd $GOPATH/src/github.com/gin-gonic/gin/examples/app-engine/`
+5. Run it: `$ dev_appserver.py .` (notice that you have to run this script by Python2)
+