Add Cron Instructions to the README

Using Cron with mutt-wizard is not intuitive for new users, since mutt-wizard requires the shell variables in `~/.profile`.

README.md

@@ -54,6 +54,15 @@ The mutt-wizard is run with the command `mw`. Once everything is setup, you'll u
 - `-f` -- Assume mailbox names and force account configuration without connecting online at all.
 - `-o` -- Configure mutt for an account, but do not keep mail offline.
 
+## Update Mailbox with Cron
+
+Since mutt-wizard requires shell variables stored in `~/.profile`, you will need to add `. $HOME/.profile` to your cronjobs. For example, to have mail update every 10 minutes, this is the line that should appear in the crontab file:
+```
+*/10 * * * * . $HOME/.profile; /usr/bin/mw -Y
+```
+
+Remember to use cron as the same user the mail is, not root. (IE, run `crontab -e`, not `sudo crontab -e`).
+
 ## Dependencies
 
 - `neomutt` - the email client.