add brief usage to doc/index.md - thanks @gabrielecirulli ❤️

doc/index.md

@@ -11,6 +11,24 @@ It's as easy as:
 
     $ npm install -g 6to5
 
+## Usage
+
+Once you've installed 6to5, there are multiple paths you can take depending on
+how you want to use it.
+
+6to5 will simply compile your ES6+ script to ES5 if you pass it as an argument
+to the command-line tool `6to5`:
+
+    $ 6to5 script.js
+
+If you have a file written using ES6+ and you just want to run it, `6to5-node`
+has you covered:
+
+    $ 6to5-node script.js
+
+And it doesn't end here! To see all the ways you can use 6to5, check out the
+[Usage](http://6to5.github.io/usage.html) page.
+
 ## [Features](features.md)
 
  - [Array comprehension](features.md#array-comprehension)