Add instructions to install via Homebrew

[samuong]

Also, I've moved the "Download Binary" method down one section, because I think
that Homebrew and Go both provide a better (automated) way to keep the binary
up-to-date.

Also, I've added http:// to http_proxy and https_proxy. This seems to be
needed by yarn.

Fixes #55

[samuong]

Oh no, I accidentally copied-and-pasted an extra space at the start of the git commit message. I'm not sure if I can live with this; is there a way to fix it?

[camh-]

git commit --amend

Your can reset the merge commit away and re merge

[samuong]

There's no merge commit - I squashed and merged it when closed the PR. I'm guessing it's not possible to force push over a protected branch, right?