add doc.go for package documentation

doc.go

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+package pubsub
+
+// The pubsub package provides facilities for the Publish/Subscribe pattern of message
+// propagation, also known as overlay multicast.
+// The implementation provides topic-based pubsub, with pluggable routing algorithms.
+//
+// The main interface to the library is the `PubSub` object.
+// You can construct this object with the following constructors
+// - `NewFloodSub` creates an instance that uses the floodsub routing algorithm.
+// - `NewGossipSub` creates an instance that uses the gossipsub routing algorithm.
+// - `NewRandomSub` creates an instance that uses the randomsub routing algorithm.
+//
+// In addition, there is a generic constructor that creates a pubsub instance with
+// a custom PubSubRouter interface. This procedure is currently reserved for internal
+// use within the package.
+//
+// Once you have constructed a `PubSub` instance, you need to establish some connections
+// to your peers; the implementation relies on ambient peer discovery, leaving bootstrap
+// and active peer discovery up to the client.
+//
+// To publish a message to some topic, use `Publish`; you don't need to be subscribed
+// to the topic in order to publish.
+//
+// To subscribe to a topic, use `Subscribe`; this will give you a subscription interface
+// from which new messages can be pumped.