docs: update build instructions and other info in readme
docs: update build instructions and other info in readme

A user new to zinc should be able to check out the code and build
the examples without having to get too lucky and/or read more of
the code than they would have liked.

Signed-off-by: Paul Osborne <b6afffd6c1a00645fdd04b09edf63d034a949a39@gmail.com>

mcoffin/zinc,mcoffin/zinc,farcaller/zinc,hackndev/zinc,posborne/zinc,troplin/zinc,farcaller/zinc,mcoffin/zinc,ntwyman/zinc,farcaller/zinc,mcoffin/zinc,richo/zinc,richo/zinc,phil-opp/zinc,richo/zinc,phil-opp/zinc,richo/zinc,phil-opp/zinc,richo/zinc,hackndev/zinc,posborne/zinc,ntwyman/zinc,troplin/zinc,phil-opp/zinc,farcaller/zinc,ntwyman/zinc,farcaller/zinc,phil-opp/zinc,troplin/zinc,hackndev/zinc,troplin/zinc,hackndev/zinc,ntwyman/zinc,ntwyman/zinc,troplin/zinc,mcoffin/zinc,posborne/zinc,posborne/zinc,hackndev/zinc,posborne/zinc