Documentation assumes homebrew package manager

[folksgl]

Only providing instructions for those who use homebrew as their package manager typically just includes those who are using Apple hardware. There are a wide range of package managers available, and in my experience the overwhelming majority of developers tend to use apt. While changing or supplementing the documentation with apt related instructions will still leave some users having to fend for themselves, the apt package manager most closely aligns with the Ubuntu-based cloud foundry runtime that this project is deployed to.