connection/README: Recommend yamux over mplex (#402)

The mplex multiplexer does not provide backpressure on a stream. Yamux does.
Given that this is a fundamental feature of a stream multiplexer, this commit
changes the spec to recommend yamux over mplex.

While yamux is not ideal, e.g. lacks backpressure on the connection level and
the number of streams, it is superior in all cases to mplex.

connections/README.md

@@ -112,8 +112,8 @@ functions.
 Each protocol supported by a peer is identified using a unique string called a
 **protocol id**. While any string can be used, the conventional format is a
 path-like structure containing a short name and a version number, separated by
-`/` characters. For example: `/mplex/1.0.0` identifies version 1.0.0 of the
-[`mplex` stream multiplexing protocol][mplex]. multistream-select itself has a
+`/` characters. For example: `/yamux/1.0.0` identifies version 1.0.0 of the
+[`yamux` stream multiplexing protocol][yamux]. multistream-select itself has a
 protocol id of `/multistream/1.0.0`. 
 
 Including a version number in the protocol id simplifies the case where you want
@@ -277,8 +277,8 @@ should support a baseline "stack" of security and multiplexing protocols.
 The recommended baseline **security** protocol is [Noise][noise-spec], which is
 supported in all current libp2p implementations.
 
-The recommended baseline **stream multiplexer** is [mplex][mplex], which
-provides a very simple programmatic API and is supported in all current libp2p
+The recommended baseline **stream multiplexer** is [yamux][yamux], which
+provides a very simple programmatic API and is supported in most libp2p
 implementations.
 
 ### State Management 
@@ -394,7 +394,7 @@ updated to incorporate the changes.
 
 [mss]: https://github.com/multiformats/multistream-select
 [uvarint]: https://github.com/multiformats/unsigned-varint
-[mplex]: ../mplex/README.md
+[yamux]: https://github.com/hashicorp/yamux/blob/master/spec.md
 [noise-spec]: ../noise/README.md
 [connmgr-v2-spec]: https://github.com/libp2p/specs/pull/161
 [connmgr-go-interface]: https://github.com/libp2p/go-libp2p-core/blob/master/connmgr/connmgr.go