[Libvncserver-common] Announcing project community chat

Hi List,

As it's 2020, the project now has a contemporary chat-based community
platform at https://gitter.im/LibVNC/libvncserver in addition to this mailing
list. The benefits are that both synchronous and async communication are
possible with this tool, it might also attract more users/devs due to having a
more "modern" feel and lower entry barrier, but that remains to be seen ;-).

There are no immediate plans to shut down this mailing list (as Gitter
requires a GitLab, GitHub or twitter account) but this might change depending
on feedback.

Let us know what you think!

Cheers,

   Christian