Homepage: Obtain pspp - MacOS - Mention homebrew installation

[Friedrich Beckmann]

Hi John,

for the MacOS system we have three different ways to install pspp

a) The DMG bundle
b) Install via macports
c) Install via homebrew (=> https://brew.sh) 

a) and b) are already mentioned here:

https://www.gnu.org/software/pspp/get.html

Can you add the homebrew install on that MacOS paragraph? You can link to the 
„tap“

https://github.com/fredowski/homebrew-pspp

which provides the build scripts for the homebrew environment.

Of course you are free to use the „badges“ showing the build status for the 
variants which I have included on the page here:

https://www.hs-augsburg.de/~beckmanf/pspp/

Cheers

Fritz