Document the current state of bank-account module.

MODULES.md

@@ -491,6 +491,60 @@ records matching ```pred?```.
 
 ### Bank Account
 
+This module creates a thin abstraction layer for bank accounts and
+transactions.
+
+    (make-bank-account number bank [transactions])
+
+* ```number``` - account number
+* ```bank``` - bank code
+* ```transactions``` - optional list of initial transactions
+
+Creates a new bank account data structure.
+
+The data structure has the following accessors:
+
+* ```(bank-account-transactions ba)``` - returns the transactions list
+* ```(bank-account-number ba)```- retrieves the bank account number
+* ```(bank-account-bank ba)``` - returns the bank account bank code
+
+The underlying implementation is currently a plain list but is subject
+to change in the future.
+
+    (bank-account-insert account transaction)
+
+* ```account``` - bank account structure
+* ```transaction``` - transaction structure to add
+
+Adds (prepends) given ```transaction``` to the list of transactions of
+given bank ```account```.
+
+    (make-bank-transaction i d a c v m t)
+
+* ```i``` - transaction id
+* ```d``` - date
+* ```a``` - amount
+* ```c``` - currency
+* ```v``` - variable symbol
+* ```m``` - message
+* ```t``` - transaction type
+
+Creates new bank transaction data structure. The underlying
+implementation is currently a record.
+
+The following accessors are provided:
+
+* ```bank-transaction-id```
+* ```bank-transaction-date```
+* ```bank-transaction-amount```
+* ```bank-transaction-currency```
+* ```bank-transaction-varsym```
+* ```bank-transaction-message```
+* ```bank-transaction-type```
+
+More fields with accessors will be added as other modules will need
+them.
+
 ### Bank Fio
 
 ### Members Payments Processor