Copy v1 Linux usermod installation step to v2

[kuhrusty]

https://docs.arduino.cc/software/ide-v1/tutorials/Linux has a "Please Read" section which talks about running usermod to give the user permission to write to the serial port. I don't see this in the v2 instructions, but it's still necessary there too; it should be copied to ide-v2-downloading-and-installing.md or (probably better) ide-v2-uploading-a-sketch.md.

[karlsoderby]

Hi @kuhrusty, we have updated the instructions for installing the IDE 2 on Linux machines. Please see this section.

[per1234]

@karlsoderby that does not resolve this request. Even when using the AppImage version of Arduino IDE 2.x, it is still necessary to give the user permissions to access the serial port in order to avoid the dreaded 'can't open device "/dev/ttyACM0": Permission denied' error.

Note that this is a different process from giving the AppImage file executable permissions.

[karlsoderby]

My bad @per1234 , I have opened a PR to add the instructions that were present in the older tutorial (#436)