Add something about the 'entity_id' parameter in the readme

Hi,

I only found out about the 'entity_type' and 'entity_id' parameters thanks to rules, when one can precise it.
To me the 'entity_id' really important, for instance to construct views based on the subjects of the transactions, and have another looks on how the points can be categorized.

I think this should be added to the readme, where the param array is broken down, just so people know they can do this kind of views easily.