README.md: add installation instruction section

Installation instructions were missing from the README, and this seemed
to be causing confusion for the users (who though they had to package
`zpm` in order to install and use it).
README.md: add a usage note to `Packaging`

Packaging is only interesting to maintainers and packagers, not end
users. So I think it's a good idea to put a small warning/note at the
top of the packaging section to keep users from falling down rabbit
holes.