Add data branch documentation for new `/bike_thefts` dataset

[tallcoleman]

File should be added bike_thefts/README.md in the data branch. A PR is not required, it can be pushed directly.

Should include:

• Link to source dataset(s)
• Brief description of key points to know (e.g. locations are nearest intersection, data is updated annually)
• Link to bikespace web page that uses the data (can point to the frontend PR in the meantime)