Create README on how to generate PS code

We should have a README.md on how to generate the code. When it was a standalone project, we could deduce that via the project file and reference to the .proto files.

Checking in the generated files is probably the right choice for this repo, but we need to document

a. where the .proto files are
b. what tools and what version of those tools were used to generate them.
c. any additional steps, like a dependency chain