docs(tonic-build): try to improve the doc-diff between tonic-build's lib.rs and Readme

[CommanderStorm]

Motivation

I had a bit of a hard time figuring out the getting started guide.
This PR tries to improve a bit on these shortcomings:

Solution

• merge the lib.rs and Readme to avoid getting to the same place in a few months
• try to reword a few of the sentences, such as explain where the example code belongs (might sound obvious to you, it is not when having not worked with build scripts that generate code..)

[CommanderStorm]

Not quite happy with the ginormous google example being in the docs at this place.
Problem: I don't know of a better solution off the top of my head..