Flesh out readme - Githubissues

Similar to https://github.com/cryptoscope/librarian/issues/2, I feel this readme can be fleshed out to make getting into go-ssb a lot easier.

I can do this, too!

Loose points & questions to answer:

how does it facilitate interaction with logs?
is it actually the append-only log (i think so)? if not, where is it? if yes, great let's mention that
how is this similar to flumedb? to kappa-core? what are the differences?
how do you get out useful data? (with multilog, which takes care of the indexing bits)
which other components does this tie into? (librarian, somehow, luigi for ingesting network data, multilog for getting out categorized data?)
log-lady pic would be :ok_hand: :man_cook:

ssbc / margaret