Add documentation for adding a new layer/point data type

[micahjohnson150]

To add a new layer or point data type, the metadata must be able to pick out the column in a csv. We need to add documentation describing this. This basically consists of documenting adding a name to: https://github.com/hpmarshall/SnowEx2020_SQLcode/blob/b4a0fb2baadedcd96fa95275c3d2262c69ed0cf4/snowxsql/metadata.py#L390