Fix bugs with the local echo channel

Allow users to change to the local echo channel without it reporting it empty and add it to README.md

README.md

@@ -22,6 +22,7 @@ The following channel prefixes exist:
 The following special channels exist:
 
  - `#main`: Normal chat without the CSM.
+ - `@`: A "local echo" channel, messages in this channel are locally displayed.
  - `@[off]`: Prefixes messages with `[off]`, displayed in chat as `[off]`.
 
 ## Added commands

init.lua

@@ -144,7 +144,7 @@ minetest.register_on_sending_chat_messages(function(msg)
             if cmdprefix ~= '#' or channels[msg:sub(2)] or msg == main_channel
               then
                 local players = chat_channels.get_channel_users(msg)
-                if players then
+                if players and msg ~= '@' then
                     local empty = true
                     for p = 1, #players do
                         if connected_players[players[p]] then