Expand generating.rst
Expand generating.rst

Add details on adding protos and yamls for project setup, example usage
for CodeGeneratorTool, and virtual environment information for running
generated code.

Change-Id: I417445b1cefad7b950209423f394b2e265e36181

googleapis/gax-python,geigerj/gax-python,geigerj/gax-python