message: Added more info on code decomposition
diff: In this guide you will see how to add a new call to Libindy. As an example we will take `indy_create_and_store_my_did` call.
  +Code will be splitted to the following layers:
  +* API layer. Library interface layer:
  + * Basic validation
  + * Conversion of C types to Rust types
  + * Propagation of execution to commands layer
  +* Commands layer:
  + * Working threads management
  + * JSONs conversion to internal types and corresponded validation
  + * Splitting complex commands to atomic operations
  + * Propagation of atomic operations execution to service layer
  + * Joining atomic operations results to complex result
  + * Execution of user defined callbacks
  +* Service layer:
  + * Implements operations business logic and complex validation
  + * Management of sockets polling threads
  +
  +
  ### API Layer
  These changes should be made in directory [`libindy/src/api`](libindy/src/api) in file `did.rs`. For your function you should choose which file it will logically fit or create a new one (first option is preferred)
  