Basic API docs of core, for JS library use

[rvagg]

"JavaScript "core" Library" in README just points to src/core package exports, we should beef this up and either auto-extract an API to document or just write notes about the most helpful ones.

I'm closing these in Synapse but to properly solve for them we'd have documentation about how you can do this easily with Filecoin Pin:

• SynapseStorage.uploadFile FilOzone/synapse-sdk#12
• SynapseStorage.uploadDirectory FilOzone/synapse-sdk#13