Update README.md

I was trying to install this plugin, I came across that using:
cordova plugin add https://github.com/m1is/AviaryCordovaPlugin
instead of plugman is the easiest way.
(for iOS you do need to install Aviary sdk as you mentioned..)
Merge pull request #2 from guysch/patch-2

Update README.md, added Cordova CLI instructions.