Added publish module documentation

doc/publish.md

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+# Publish system overview
+
+The publish module intend to be a "multi-pin" feature for chats, allowing users to retrieve importants messages in a crowded conversation.
+
+## Commands
+
+
+### Subscribe
+
+**Description** : Subscribe to this group chat publications
+
+**Usage location** : Group chat only
+
+**Command syntax** : /subscribe
+
+
+### Unsubscribe
+
+**Description** : Unsubscribe [target chat member] from this group chat publications. If no member is provided, unsubscribe yourself.
+
+**Usage location** : Group chat
+
+**Command syntax** : /unsubscribe [target chat member]
+
+*note* : parameter [target chat member] not implemented
+
+
+### ListSubscribers
+
+**Description** : List all subscribers of this group chat
+
+**Usage location** : Group chat
+
+**Command syntax** : /listsubscribers
+
+
+### Publish
+
+**Description** : Publish a message to Alfred for this group chat and sends this message via MP to every subscriber of the group chat. The message can then be retrieved using the *retrieve* command.
+
+**Usage location** : Group chat only
+
+**Usage conditions** : Reply to the message to publish
+
+**Command syntax** : /publish
+
+*note* : MP not implemented
+
+### Unpublish
+
+**Description** : Remove a published message from Alfred for this group chat
+
+**Usage location** : Group chat only
+
+**Command syntax** : /unpublish [publication ID]
+
+
+### Retrieve
+
+**Description** : if MP, retrieve all messages from subscribed sources for user, if Group Chat, retrieve all published messages for this chat 
+
+**Usage location** : Group Chat or MP
+
+**Command syntax** : /retrieve
+