Documentation: Improve wording on requirements for a free Nitrokey

Documentation: Improve wording on requirements for a free Nitrokey

"listed in MAINTAINERS" is not enough to qualify for the free Nitrokey
Start. You have to be listed in an M: entry. Mention that to reduce
confusion for reviewers who wonder why their application fails.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251203074349.1826233-2-u.kleine-koenig@baylibre.com>