[#170] storagegroup: Refactor and document package functionality

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

storagegroup/doc.go

@@ -0,0 +1,36 @@
+/*
+Package storagegroup provides features to work with information that is
+used for proof of storage in NeoFS system.
+
+StorageGroup type groups verification values for Data Audit sessions:
+	// receive sg info
+
+	sg.ExpirationEpoch() // expiration of the storage group
+	sg.Members() // objects in the group
+	sg.ValidationDataHash() // hash for objects validation
+	sg.ValidationDataSize() // total objects' payload size
+
+Instances can be also used to process NeoFS API V2 protocol messages
+(see neo.fs.v2.storagegroup package in https://github.com/nspcc-dev/neofs-api).
+
+On client side:
+	import "github.com/nspcc-dev/neofs-api-go/v2/storagegroup"
+
+	var msg storagegroup.StorageGroup
+	sg.WriteToV2(&msg)
+
+	// send msg
+
+On server side:
+	// recv msg
+
+	var sg StorageGroupDecimal
+	sg.ReadFromV2(msg)
+
+	// process sg
+
+Using package types in an application is recommended to potentially work with
+different protocol versions with which these types are compatible.
+
+*/
+package storagegroup