Docs for Addon installation and configuration

Please consider adding an Installation/Configuration page to the docs under **Addons** (above _Building Addons_, _API Reference_).

I didn't find instructions under _Getting Started_, _Control Panel_, or _Addons_. The only place I saw directions was in the _Marketplace_ when viewing an Addon. I was curious if I could install them via the command-line, but it appears not.
