[Docs] Write Files API Documentation

[flyingzumwalt]

Explore the Files API and write documentation for the Primer. The most current explanation of the files API is in the discussion here: ipfs/specs#98

cc @RichardLitt

[flyingzumwalt]

If people have other reference info to offer, please let me know. cc @diasdavid @whyrusleeping

[daviddias]

Can we just use ipfs/specs#88 to track this? Closing, reopen if you see necessity in having two issues.

[RichardLitt]

This isn't about the specs, which is a separate issue, but about documenting it with examples. Much of the work for this is being done in https://github.com/flyingzumwalt/decentralized-web-primer by @flyingzumwalt now, I believe.

[daviddias]

Now on https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/FILES.md