Add warnings about breaking changes in network (#2348)

substrate/core/network-libp2p/src/lib.rs

@@ -15,6 +15,9 @@
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
 //! Networking layer of Substrate.
+//! 
+//! **Important**: This crate is unstable and the API and usage may change.
+//!
 
 mod behaviour;
 mod config;

substrate/core/network/src/lib.rs

@@ -19,6 +19,9 @@
 
 //! Substrate-specific P2P networking: synchronizing blocks, propagating BFT messages.
 //! Allows attachment of an optional subprotocol for chain-specific requests.
+//! 
+//! **Important**: This crate is unstable and the API and usage may change.
+//!
 
 mod service;
 mod sync;