Readme.md. Add Logging section

Author: andrykonchin

README.md

@@ -914,6 +914,32 @@ Dynamoid.configure do |config|
 end
 ```
 
+## Logging
+
+There is a config option `logger`. Dynamoid writes requests and
+responses to DynamoDB using this logger on the `debug` level. So in
+order to troubleshoot and debug issues just set it:
+
+```ruby
+class User
+  include Dynamoid::Document
+  field name
+end
+
+Dynamoid.config.logger.level = :debug
+Dynamoid.config.endpoint = 'localhost:8000'
+
+User.create(name: 'Alex')
+
+# => D, [2019-05-12T20:01:07.840051 #75059] DEBUG -- : put_item | Request "{\"TableName\":\"dynamoid_users\",\"Item\":{\"created_at\":{\"N\":\"1557680467.608749\"},\"updated_at\":{\"N\":\"1557680467.608809\"},\"id\":{\"S\":\"1227eea7-2c96-4b8a-90d9-77b38eb85cd0\"}},\"Expected\":{\"id\":{\"Exists\":false}}}" | Response "{}"
+
+# => D, [2019-05-12T20:01:07.842397 #75059] DEBUG -- : (231.28 ms) PUT ITEM - ["dynamoid_users", {:created_at=>0.1557680467608749e10, :updated_at=>0.1557680467608809e10, :id=>"1227eea7-2c96-4b8a-90d9-77b38eb85cd0", :User=>nil}, {}]
+```
+
+The first line is a body of HTTP request and response. The second line -
+Dynamoid internal logging of API call (`PUT ITEM` in our case) with
+timing (231.28 ms).
+
 ## Credits
 
 Dynamoid borrows code, structure, and even its name very liberally from the truly amazing [Mongoid](https://github.com/mongoid/mongoid). Without Mongoid to crib from none of this would have been possible, and I hope they don't mind me reusing their very awesome ideas to make DynamoDB just as accessible to the Ruby world as MongoDB.