added deatils on logging with in grabbit to documentation

Author: Guy Baron

README.md

@@ -20,7 +20,7 @@ A lightweight transactional message bus on top of RabbitMQ supporting:
 4) Publisher confirms
 5) [Reliable messaging](https://github.com/wework/grabbit/blob/master/docs/OUTBOX.md) and local service transactivity via Transaction Outbox pattern
 6) Deadlettering
-7) Structured logging
+7) [Structured logging](https://github.com/wework/grabbit/blob/master/docs/LOGGING.md)
 
 Planned:
 
@@ -79,7 +79,7 @@ Register a command handler
 ```Go
 
 
-handler := func(invocation gbus.Invocation, message *gbus.BusMessage) error
+handler := func(invocation gbus.Invocation, message *gbus.BusMessage) error{
     cmd, ok := message.Payload.(*SomeCommand)
     if ok {
       fmt.Printf("handler invoked with  message %v", cmd)

docs/LOGGING.md

@@ -0,0 +1,22 @@
+# Logging
+
+grabbit supports structured logging via the [logrus](https://github.com/sirupsen/logrus) logging package.
+The logger is accessible to message handlers via the past in invocation instance.
+
+```go
+
+func SomeHandler(invocation gbus.Invocation, message *gbus.BusMessage) error{
+    invocation.Log().WithField("name", "rhinof").Info("handler invoked")
+    return nil
+  }
+
+```
+
+grabbit will create a default instance of logrus FieldLogger if no such logger is set when the bus is created.
+In order to set a custom logger when creating the bus you need to call the Builder.WithLogger method passing it
+a logrus.FieldLogger instance.
+
+```go
+
+
+```