Add README for google-api-go-generator

[quartzmo]

google-api-go-generator needs a usage README.

Here's some possible content:

How to manually generate Apiary from JSON not in discovery doc

1. In google-api-go-client/google-api-go-generator, run go build. It should produce a google-api-go-generator binary.
2. Generate clients from cached discovery documents:

   ./google-api-go-generator -install -api_json_file=../test.json -gendir=..
   

   Or, download a discovery doc and generate a single client:

   ./google-api-go-generator -cache=false -install -api=aiplatform:v1beta1 -gendir=.