Mention channel/key bug in irkerd docs

Created by: vimalloc

Per this issue: https://gitlab.com/esr/irker/issues/2

A documentation update was added to irkerd (https://gitlab.com/esr/irker/commit/190808c37d4ab5f0f16fe35352ff36863c2732d5) but the bug is still there. Making a note of it here could save someone a lot of hassle.

This could probably be worded better if someone else wants to take a stab at it.