Add Buildpack Instrunctions #71

Other frameworks have buildpack instructions for easily using this library in a container.

• https://github.com/GoogleCloudPlatform/functions-framework-nodejs#quickstart-build-a-deployable-container
• https://github.com/GoogleCloudPlatform/functions-framework-go#go-further-build-a-deployable-container
• https://github.com/GoogleCloudPlatform/functions-framework-python#quickstart-build-a-deployable-container

It would be nice if we had simple instructions no how to configure this for Java.