Add Common Problems to README

[JamesXNelson]

It can be helpful if common problems users may encounter are included in the readme (like having to run $ gcloud components update app-engine-java first).

My gotcha was when I tried to adapt an existing appengine project to use managed vms, and my pom lacked <packaging>war</packaging>. It spit out:

Error Response: [400] “env” setting is not supported for this deployment

Took me a while commenting out giant swaths of my app to finally notice the difference.

Cheers.