Maintainers.txt: update for filesystem area descriptions

Maintainers.txt: update for filesystem area descriptions

Add comment describing new F: and X: tags for associating maintainership
sections with specific filesystem paths, including wildcards.

Add global section associating *all* code with devel@edk2.groups.io,
with a default '*' F: tag directing all modifications that do not hit a
rule to the stewards.

Also tag all files in top directory as maintained by the stewards.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>