feat: multistream-select (#297)

Author: salmad3

content/concepts/introduction/protocols/multistream.md

@@ -1,7 +1,23 @@
 ---
 title: "Multistream Select"
-description: ""
+description: "Multistream Select is used to negotiate the protocol to be spoken on a connection or stream."
 weight: 26
 ---
 
-Coming soon!
+## Overview
+
+[Multistream Select](https://github.com/multiformats/multistream-select) is a
+protocol negotiation protocol. It allows two peers to negotiate the next spoken
+protocol on a connection or stream.
+
+Peers use Multistream Select at various places to select the protocols to use on
+a libp2p connection or stream. Most prominently Multistream Select is used to
+select the [security](../../secure-comm/overview.md) followed by [stream
+multiplexing](../../multiplex/overview.md) protocol spoken on a connection. In
+addition it is used to select the application protocol (e.g. Kademlia) on each
+new stream on a given connection. The [protocol
+negotiation](../core-abstractions/connections#protocol-negotiation) and [upgrade
+process](../core-abstractions/connections#upgrading-connections) are explained
+further in the [connections document](../core-abstractions/connections.md).
+
+{{< alert icon="💡" context="note" text="See the multistream-select <a class=\"text-muted\" href=\"https://github.com/multiformats/multistream-select\">technical specification</a> for more details." />}}