README: Note that user groups don't get updated until re-login

README

@@ -208,6 +208,8 @@ To use FPGAs, you will need to be sure the user BFGMiner is running as has
 appropriate permissions. This varies by operating system.
 On Gentoo: sudo usermod <username> -a -G uucp
 On Ubuntu: sudo usermod <username> -a -G dialout
+Note that on GNU/Linux systems, you will usually need to login again before
+group changes take effect.
 
 By default, BFGMiner will scan for autodetected FPGAs unless at least one
 -S is specified for that driver. If you specify -S and still want BFGMiner