Add a breakdown of the structure of `Transaction` in the wiki

[nazrinharris]

Basically list and explain the assumptions I'm making in order to make things work. For example, in a credit transaction, base is the outside source, while target is the account itself. For debit transactions, base is inside source (i.e. the account itself) and target is the outside destination.

[nazrinharris]

Done, could be improved later on though.