documentations on schema.json and datamapper.json

[leichuan]

It would be good to explain some of the key fields in both schema.json and datamapper.json files. For example, in datamapper.json, are Left/Right in edgeMap section specific to janusgraph-utils? Or it should be consistent with the column names in the actual csv files. In schema.json, how these fields (e.g., partition, composite, etc.) relate to the configurations in JanusGraph? With some brief descriptions, the learning process can be much easier.