devapp: update README.md to add the installation of App Engine SDK
devapp: update README.md to add the installation of App Engine SDK

Adds a step for the first time setup section which installs the App
Engine SDK.

Change-Id: I9aef3695e1e8e52a3bdb95caa898dfc0d9b70e8c
Reviewed-on: https://go-review.googlesource.com/c/build/+/208338
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build