IRC bridge: Add documentation on nick password.
`--stream` is a Zulip stream.
`--topic` is a Zulip topic, is optionally specified, defaults to "IRC".
+`--nickserv-pw` is the IRC nick password.
IMPORTANT: Make sure the bot is subscribed to the relevant Zulip stream!!
Specify your Zulip API credentials and server in a ~/.zuliprc file or using the options.
-Note that "_zulip" will be automatically appended to the IRC nick provided
+IMPORTANT: Note that "_zulip" will be automatically appended to the IRC nick provided, so make sure that your actual registered nick ends with "_zulip".
## Example
