Add information about help channel DM service command to help channel guide

[SebastiaanZ]

We recently added a command, !helpdm on, to the bot that allows users to enable notifications for the help channel of others that they are active in. However, the command is currently only mentioned in a #changelog entry, which scrolls out of view rather quickly.

@MarkKoz suggested that we should add it to the help channel guide, which is a good idea.

[jchristgit]

@minalike will take care of this in #624