Add some info about concurrent usage to client mod (#328)

tonic/src/client/mod.rs

@@ -6,6 +6,15 @@
 //!
 //! This client is generally used by some code generation tool to provide stubs
 //! for the gRPC service. Thusly, they are a bit cumbersome to use by hand.
+//!
+//! ## Concurrent usage
+//!
+//! Upon using the your generated client, you will discover all the functions
+//! corresponding to your rpc methods take `&mut self`, making concurrent
+//! usage of the client difficult. The answer is simply to clone the client,
+//! which is cheap as all client instances will share the same channel for
+//! communication. For more details, see
+//! [transport::Channel](../transport/struct.Channel.html#multiplexing-requests).
 
 mod grpc;
 mod service;