Add README file with description of demo impl.

README.md

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+# Bank account reactive DDD DEMO
+This is the DDD reactive implementation of bank account example. The purpose of
+this demo is to demonstrate how to deal with data consistence in the reactive implementation
+using DDD (domain driven design) principles.
+
+## List of design patterns used in this demo
+* DDD
+* message driven architecture
+* event sourcing
+* CQRS
+
+## Technologies
+This demo is implemented using Spring reactor project.
+
+## The implementation consists by two aggregates 
+* `Account`
+* `Transaction`
+
+Those two aggregates representing one bounded context `Bank account`.
+
+## Supported operation by Account aggregate
+`account-domain-api` module contains definition of contract provided by `Account` aggregate.
+Here is the list of basic operations:
+* create account                        
+* deposit money                         
+* withdraw money                        
+* transfer money between two accounts 
+ 
+## Supported operation by Transaction aggregate
+`transaction-domain-api` module contains definition of contract provided by `Transaction` aggregate
+Here is the list of basic operations:
+* create transaction
+* cancel/rollback transaction
+* finish transaction 
+
+## Persistence
+Implementation is following CQRS and event sourcing design pattern. Persistence API is provided by
+`common-persistence-api`. This module provides in memory storage for:
+ * events generated by particular aggregate `AggregateRepositoryImpl` 
+ * storage for view model `ViewRepositoryImpl`
+ 
+## Business case representing of data consistency
+ * Create two accounts `A` and `B` with specific amount of money
+ * Create the transaction for transferring specific amount of money from account `A` to account `B`
+ * Transaction finished: successfully or failed
+
+### Transaction is successful
+ * Transaction closes and informs about result. 
+### Transaction failed:
+#### Failed due to account `A` doesn't have enough amount
+ * Transaction closes and informs about result.
+#### Failed due to account `B` doesn't exist
+ * Transaction returns money to account `A`
+ * Transaction closes and informs about result.
+#### Failed due to cancellation 
+ * If money were withdrawn from account `A` -> money returns to account `A`
+ * Transaction closes and informs about cancellation.
+