Add installation instructions

[schmunk42]

I saw the clients from http://files.puphpet.com/cli/ and it's working fine on my machine, but it would be nice to have a preferred way of installing them.
Could be similar to https://github.com/composer/composer#installation--usage maybe.

What's about making them available with system package managers, like apt-get, brew or macports?

[rider76]

But http://files.puphpet.com is offline, this app no ​​longer works