Add explanation of howler distribution files

Fixes #722

README.md

@@ -90,6 +90,13 @@ import {Howl, Howler} from 'howler';
 const {Howl, Howler} = require('howler');
 ```
 
+Included distribution files:
+
+* **howler**: This is the default and fully bundled source the includes `howler.core` and `howler.spatial`. It inclues all functionality that howler comes with.
+* **howler.core**: This includes only the core functionality that aims to create parity between Web Audio and HTML5 Audio. It doesn't include any of the spatial/stereo audio functionality.
+* **howler.spatial**: This is a plugin that adds spatial/stereo audio functionality. It requires `howler.core` to operate as it is simply an add-on to the core.
+
+
 ### Examples
 
 ##### Most basic, play an MP3: