Document what checks are performed

[glenjamin]

Requested Update

It would be great if the README could give an overview of what checks are actually being performed - does es-check only look at syntax, or does it also attempt to look for access to library functions from newer ES versions?

Why Is This Update Needed?

It's unclear what categories of checks the tool attempts to perform

Are There Examples Of This Requested Update Elsewhere?

#37 is slightly related, as it's unclear whether that's a bug or just an intentional ommission