Update documentation about card deduplication to make it clear the order of deduplication.

A note from @ehk20 that is worth clarifying within the readme:

Useful note (possibly for the documentation) the order of the deduplication (if you are using extra-fields, for example) seems to be that the first returned record is the one that is preserved so you may want to order your queries by priority accordingly. ie. you may want a hit on group, being more specific than INST, to be at the top.