Add note: replace localhost with server_name
Add note: replace localhost with server_name

IMHO it was not clear that localhost is the servername.
(Because in the note which says that the AS user must be invited if the room ist non-public the example userid is `@slackbot:domain` not `:localhost`)
matrix-org/matrix-appservice-bridge,matrix-org/matrix-appservice-bridge,matrix-org/matrix-appservice-bridge