Explicitly mention availability of brew installation in docs

[neilotoole]

There's a brew formula available for mage: it should be explicitly mentioned in the repo README.md and on magefile.org.

It really makes life easier to sell the usage of mage to skeptics if you can say to do this:

$ cd my/repo
$ brew install mage
$ mage install

Previously I'd been go-getting mage and using bootstrap.go etc... that was more effort than required (it's so simple with brew) and I saw pushback against the use of mage for that reason. I didn't know there was a brew formula. It's not in the docs, or at least, not obvious.

The website docs and README.md should be updated to reflect how easy it is to install mage via brew.

P.S. mage is wonderful. I was a minor skeptic initially, but I'm sold. Well done.

Related: #234

[natefinch]

Ahh, cool, I honestly didn't know there was a brew formula for it :) I should add that to how we release, and yeah, definitely add it to the docs.