Add note about mqtt special chars when using uplink.

[derfloh]

What did you change

Add a note about using problematic chars in channel names when using MQTT.

Why did you change it

I had a hard time figuring out why DMs and messages to LongFast where published to my MQTT server, but not messages to "Channel#1" - After spending a couple of hours on various things like factory reset, reflashing, messing up my config even further etc. I finally realized that my messages couldn't be published because my Channel Name was incompatible with MQTT. I hope that adding this note will prevent others going down the same rabbit hole as I did.

[vercel[bot]]

@derfloh is attempting to deploy a commit to the Meshtastic Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}