Update README.md

add install cmds for NS 3+ and <3

Author: bradmartin

README.md

@@ -21,8 +21,12 @@ Android Sample |  iOS Sample
 ## Installation
 From your command prompt/terminal go to your app's root folder and execute:
 
+#### NS 3.0+
 `tns plugin add nativescript-checkbox`
 
+#### NS < 3.0
+`tns plugin add [email protected]`
+
 ## Usage
 
 ###