This commit basically does two things: it urges users always to use an own
git repository to manage the site configuration, and it removed all
mentions of "communities" from the "Getting started" page, as it was too
Freifunk-specific.

Other documentation pages (especially the site configuration docs) should
probably be adjusted as well.

`make modules` will generate a opkg repository containing kernel modules
matching the kernel of the Gluon images.

We'll soon have other output besides the images, having a single directory
for all output makes sense.

Build requires Python 2 and libssl-dev

noob-proofing a bit

add information about the environment variables.