Added a noun and added a serial (AKA Oxford) comma

Nitpicky grammar stuff, but every little thing counts. I realize that the sentence could technically by correct as 

> Homestead runs on any Windows, Mac, [...]

by using the OS names as the nouns, and implying it works on any version, but the conjunction should then be "or," not "and." In either case, I think 

> Homestead runs on any Windows, Mac, or Linux system [...]

rolls better. 

The serial comma just brings it into line with the rest of the documentation (no other files lack a serial comma).