Start writing about financial entities

Created by: eileenmcnaughton

I have added the start of an initial page explaining the various financial entities. It's incomplete but I'm still with the idea we should merge fast & edit lots in this evolving code area.

One thing that confused me a lot is that I think the sample data financial_trxn to & from accounts may not be correct

@JoeMurray @artfulrobot @monishdeb @kcristiano @MikeyMJCO