Instructions - add info about adding user to `dialout` group

[tstriker]

On linux in order for chromium to access the serial ports, the user has to belong to the dialout group:

z ~$ ls -ld /dev/ttyACM0 
crw-rw----. 1 root dialout 166, 0 Mar  2 14:55 /dev/ttyACM0

And at least in fedora that is not the default case. So, for the demo to work, need to add user to the dialout group:
sudo usermod -a -G dialout <username>

and then restart the session (my linux chops are not strong enough so i just reboot but i'm sure there is a more direct command to that effect; logging out and back in doesn't do it)

after which the demo works!

[brooksvb]

On Arch Linux the group is uucp

❯ ls -ldg /dev/ttyACM*
crw-rw---- 166,0 root uucp 24 Apr 19:33 /dev/ttyACM0
crw-rw---- 166,1 root uucp 24 Apr 19:33 /dev/ttyACM1

Adding my user to the group did the trick, but you must log out and log back in for it to take effect.