Update documentation to reflect deprecation of "NewEnvClient"
As `NewEnvClient` is deprecated in favor of `NewClientWithOpts`, the main package documentation should reflect this. This is also the text that appears on godoc.org so it's quite important that it is correct (for newbies like me)
@@ -7,8 +7,8 @@ https://docs.docker.com/engine/reference/api/
Usage
You use the library by creating a client object and calling methods on it. The
-client can be created either from environment variables with NewEnvClient, or
-configured manually with NewClient.
+client can be created either from environment variables with NewClientWithOpts(client.FromEnv),
+or configured manually with NewClient().
For example, to list running containers (the equivalent of "docker ps"):
