Update README install steps

This also includes some cosmetic cleanup.

Additional context is provided with individual commit messages.
It's also not strictly true. You only have to have gpg if you want crypto signing of your certs to work. You could turn verification off, or unset your gpg key id, and it will just skip those bits.

A quick, "brew install gnupg if you're on a mac, or apt-get install gnupg on ubuntu" is theoretically all that's needed, but of course as soon as you write that someone running FreeBSD will send a bitchy email asking about merge or whatever. I don't think this belongs at the very top, but probably pretty close to it.

@stvstnfrd if you do the minimal thing of "hey you need to install gnupg, however that works", I'll come in after you and add a kind of what-is-this-repo-what-can-it-do, and then I'll make the gnupg thing a caveat to the "it can crypto sign your pdf!" bit. 

Maybe bump it up to about a quarter down from the top?