Wrote readme

Readme.md

@@ -5,3 +5,23 @@ Respond with style
 This application provides `cowboy` `stream_handler` to send customized response bodies
 depending on status code. For every response that cowboy is about to send, `cowboy_kitten` will
 check status code against rules, you set and modify response body if nessesary.
+
+## Usage
+
+Just add cowboy_kitten to the stream chain. It is recomended to add it just before the `cowboy_stream_h` handler. `resp_bodies` should be provided inside the `env` map, that is a part of cowboy setup.  
+#### Response body format
+Response bodies can be one of two types: files or binary strings, if you want to send file, use `{file, Filename}` tuple, binary strings can be used directly.
+
+### Example
+ ```
+cowboy:start_clear(http, [{port, 8080}], #{
+    stream_handlers => [cowboy_kitten, cowboy_stream_h],
+    env => #{
+        dispatch => Dispatch,
+        resp_bodies => #{
+            500 => {file, "500_error_resp_file"},
+            400 => <<"400_error_msg">>
+        }
+    }
+}).
+ ```