Add a crate doc comment with link to Yamux spec.
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
+//! Implements the Yamux multiplexing protocol for libp2p, see also the
+//! [specification](https://github.com/hashicorp/yamux/blob/master/spec.md).
+
extern crate bytes;
extern crate futures;
#[macro_use]
