Add information about system-wide install

Even though I'm familiar with `rbenv`, but I was confused a little bit because `phpenv` is nothing but tiny wrapper of `rbenv` entirely.

I felt like what is `PHPENV_ROOT` and how `RBENV_ROOT` does a meaningful for `phpenv` . It is simple to understand, and I love the way what you selected.

Since `phpenv` is a great tool, it's not for only development tool but production use.
So I add tiny tips to README how to install `phpenv` as system-wide.

I'm not sure whether you feel it's not the way as you intended, then it's OK.
Thanks!