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)

Signed-off-by: Tobias Pfandzelter <pfandzelter@campus.tu-berlin.de>

client/client.go

@@ -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"):