Add link to Formula on marketing page

Me every time I want to know if Homebrew has a given package:
- http://lmgtfy.com/?q=homebrew+mac&l=1
- Click: "Fork me on GitHub"
- Click: "Library"
- Click: "Formula"

Now we have a one-click step to discovering what Homebrew can
install direct from the homepage. W00t!

Closes #18349.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>