docs: extend FAQ

Author: zsquareplusc

documentation/appendix.rst

@@ -71,6 +71,14 @@ User supplied URL handlers
     search path in :data:`serial.protocol_handler_packages`. This is possible
     starting from pySerial V2.6.
 
+``Permission denied`` errors
+    On POSIX based systems, the user usually needs to be in a special group to
+    have access to serial ports.
+
+    On Debian based systems, serial ports are usually in the group ``dialout``,
+    so running ``sudo adduser $USER dialout`` (and logging-out and -in) enables
+    the user to use the port.
+
 
 Related software
 ================