Add "permission denied" FAQ entry (#2564)

docs/FAQ.md

@@ -41,6 +41,18 @@ when using a serial port, the monitor baud rate can be set to 4800 with the foll
 
 `$ arduino-cli monitor -p <port> --config baudrate=4800`
 
+## "Permission denied" error in sketch upload
+
+This problem might happen on some Linux systems, and can be solved by setting up serial port permissions. First, search
+for the port your board is connected to, with the command:
+
+`$ arduino-cli board list`
+
+Then add your user to the group with the following command, replacing `<username>` with your username and `<group>` with
+your group name. Logging out and in again is necessary for the changes to take effect.
+
+`$ sudo usermod -a -G <group> <username>`
+
 ## Additional assistance
 
 If your question wasn't answered, feel free to ask on [Arduino CLI's forum board][1].