Further improve README

[tatiana]

There are still a couple of improvements that we could make after #39, which can
be addressed in a separate PR:

• links to examples and code could be embedded, as opposed to links
• we need to do more work on the quickstart - allowing users to try out
  with minimum setup, steps and an example that would work for anyone
  out-of-the-box
• render the supported databases/object stores in a more user-friendly
  way