Document new ColumnMapping format

Need to add documentation for the ColumnMapping format. Relevant points:

• JSON array of mapping definitions
• Mapping definition is a JSON object with required "type", "source" attributes

Types:
column: requires "family" and "qualifier", regular storage
counter: requires "family" and "qualifier", incrementable storage, must be int/long
keyAsColumn: requires "family", optional "prefix", must be record or map type, maps each value to a separate qualifier in the same family
key: gets value from storage in the key. Primitive types only.
occVersion: occ version, must be an int/long and conflicts with counter types (needs more doc on occ)