Add documentation format of environment variable

This fixes #277

Author: samos123

README.md

@@ -78,6 +78,9 @@ to those for the `docker run` command except it doesn't support the
 attach options (`-a`). See "Port bindings" and "Using volumes" below for
 more information on how to create port bindings and volume mappings.
 
+The `environment` variable accepts a dictionary or a list of strings
+in the following format `["PASSWORD=xxx"]` or `{"PASSWORD": "xxx"}`.
+
 `volumes_from` and `dns` arguments raise TypeError exception if they are used
 against v1.10 of docker remote API. Those arguments should be passed to
 `start()` instead.