This PR documents functionality added by
https://github.com/freifunk-gluon/packages/pull/95

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

 - generalize, to make it more clear that the site config
   needs to be adapted to the local community.
 - remove some host names and public keys used in Lübeck
 - suggest private space mac adresses
 - change fastd method to salsa2012+umac, which is the recommended default
 - add config for a fastd peer reachable by both ipv4 and ipv6
 - order GLUON_SITE_PACKAGES alphabetically

- DIR 615 E1
+ CPE210
+ CPE510

Link to timezone configuration help had a wrong anchor

More information on how to get correct image file naming.

noob-proofing a bit