[#105] netmap: Add docs

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

netmap/doc.go

@@ -1,2 +1,34 @@
-// Netmap contract description.
+/*
+Netmap contract is a contract deployed in NeoFS side chain.
+
+Netmap contract stores and manages NeoFS network map, Storage node candidates
+and epoch number counter. In notary disabled environment, contract also stores
+list of Inner Ring node keys.
+
+Contract notifications
+
+AddPeer notification. This notification is produced when Storage node sends
+bootstrap request by invoking AddPeer method.
+
+  AddPeer
+    - name: nodeInfo
+      type: ByteArray
+
+UpdateState notification. This notification is produced when Storage node wants
+to change it's state (go offline) by invoking UpdateState method. Supported
+states: (2) -- offline.
+
+  UpdateState
+    - name: state
+      type: Integer
+    - name: publicKey
+      type: PublicKey
+
+NewEpoch notification. This notification is produced when new epoch is applied
+in the network by invoking NewEpoch method.
+
+  NewEpoch
+    - name: epoch
+      type: Integer
+*/
 package netmap