[#227] netmap: Refactor and document package functionality

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

netmap/doc.go

@@ -1,11 +1,42 @@
 /*
-Package netmap provides routines for working with netmap and placement policy.
-Work is done in 4 steps:
-1. Create context containing results shared between steps.
-2. Processing filters.
-3. Processing selectors.
-4. Processing replicas.
+Package netmap provides functionality for working with information about the
+NeoFS network, primarily a layer of storage nodes.
+
+The package concentrates all the characteristics of NeoFS networks.
+
+NetMap represents NeoFS network map - one of the main technologies used to
+store data in the system. It is composed of information about all storage nodes
+(NodeInfo type) in a particular network. NetMap methods allow you to impose
+container storage policies (PlacementPolicy type) on a fixed composition of
+nodes for selecting nodes corresponding to the placement rules chosen by the
+container creator.
+
+NetworkInfo type is dedicated to descriptive characterization of network state
+and settings.
+
+Instances can be also used to process NeoFS API V2 protocol messages
+(see neo.fs.v2.netmap package in https://github.com/nspcc-dev/neofs-api).
+
+On client side:
+	import "github.com/nspcc-dev/neofs-api-go/v2/netmap"
+
+	var msg netmap.NodeInfo
+	msg.WriteToV2(&msg)
+
+	// send msg
+
+On server side:
+	// recv msg
+
+	var info netmap.NodeInfo
+
+	err := info.ReadFromV2(msg)
+	// ...
+
+	// process dec
+
+Using package types in an application is recommended to potentially work with
+different protocol versions with which these types are compatible.
 
-Each step depends only on previous ones.
 */
 package netmap