batman-adv: README cleanups

- Add routing_algo

- Remove date from README:
The date has to be updated when a patch touches the README. Therefore, nearly
every feature will modify this date. It can happens quite often that not only
one feature is currently in development or waiting on the mailinglist. This
creates merge conflicts when applying a patchset.

The date itself doesn't provide any additional information when this file is
only available in a release tarball or as part of a SCM repository.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>