Readme updates

[jono-poltrack]

Update readme so it has

1. Instructions if you do not use (or use a modified version) of the apiwrapper.js scorm file
2. Update the limitations section once more data model elements are added - Fully implement SCORM data model #13
3. Instructions for how to configure via the manifest vs. manually - Initialization of "launch_data" is the only way to set xapi mandatory info #15