docs: Improve README.md to tell people how to install Shrinkwrap

README.md was previously not very clear for newcommers, so make it clear
that the documentation contains a quickstart guide, and also add an even
quicker guide in-place for the lazy.

We also remove the instructions for building the documentation locally
and move it to the readthedocs documenation.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>