Update README

[gmaclennan]

The readme is out-of-date (is for v2). Needs updating, low priority.

[achou11]

Running list of things that should be documented:

• exported JSONSchema objects (https://github.com/digidem/mapeo-schema/issues/87#issuecomment-1668238615)
• encode/decode functions
• validate function
• get/parse version id functions (https://github.com/digidem/mapeo-schema/pull/100)