Add /data/api description under /data/README.md

[CharlieJCJ]

Currently the purpose of api is slightly unclear in /data/ and root README.

Also need s/api/<a disambiguous name>

[ShishirPatil]

This was the raw sources of the API. I thought it might be useful for folks who might want to use this to re-generate self-instruct data for either SFT (training) or eval to improve on the APIBench dataset.