doc: explain how to run App Engine version locally
doc: explain how to run App Engine version locally

This should give contributors who are unfamiliar with Google App Engine
enough information to get started.

Change-Id: I028b31f283443b6d98e5a397c06ba1b46afe3ca3
Reviewed-on: https://go-review.googlesource.com/15653
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

scryner/tour,shuLhan/tour,dupoxy/go-tour-fr,golians/tour,Go-zh/tour,Go-zh/tour,golang/tour,golians/tour,scryner/tour,scryner/tour,zorion/go-tour-ca,dupoxy/go-tour-fr,shuLhan/go-tour-id,scryner/tour,shuLhan/go-tour-id,Go-zh/tour,dupoxy/go-tour-fr,Go-zh/tour,shuLhan/tour,golians/tour,shuLhan/go-tour-id,shuLhan/tour,zorion/go-tour-ca