Add per-platform instructions? First-run setup could use a lot of UX improvement

I found the first-run setup of Keybase to be insanely complicated. I'm very well-versed in working in the command-line, I've got a working `homebrew` install, an up-to-date `npm` install, etc etc… but I still found several points in the installation instructions where I had to simply figure out how to accomplish something Keybase needed set up.

To be fair, I think a lot of this is platform-specific, but if we want to have Keybase be fairly turn-key to setup, a good per-platform installation guide is key. (Think: links and one-liners for installing things like `homebrew` (needed to get `gpg` installed), etc.)

I'll update this issue with notes from a meeting about Keybase I'm having with a couple of friends this weekend.

Review the existing first-run installation instructions and identify the platform-specific setup steps mentioned in the issue, including Homebrew, gpg, and npm prerequisites. Done means providing clear per-platform installation guidance with relevant links and one-liners; the issue names no files or tests to run.

Written by the indexing model from the issue text.