Add note about user namespaces to linux doc

needed for #1986

mostly ported over from
https://github.com/brave/browser-laptop/blob/master/docs/linuxInstall.md

docs/source/installing-brave.rst

@@ -4,6 +4,11 @@ Installing Brave
 Linux
 *****
 
+NOTE: If Brave does not start and shows an error about sandboxing, you may need
+to enable `user namespaces
+<https://superuser.com/questions/1094597/enable-user-namespaces-in-debian-kernel#1122977>`_. For security reasons, we do NOT recommend running with the ``--no-sandbox`` flag. For more info, see https://github.com/brave/brave-browser/issues/1986#issuecomment-445057361.
+
+
 Release Channel Installation
 ============================