[conformal/btcdb] fbb17f: Update TODO section of README.md.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcdb
  Commit: fbb17fd35a9416ddba91d980a1a6d3b9fe9a4c8d
      
https://github.com/conformal/btcdb/commit/fbb17fd35a9416ddba91d980a1a6d3b9fe9a4c8d
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-07-07 (Mon, 07 Jul 2014)

  Changed paths:
    M README.md

  Log Message:
  -----------
  Update TODO section of README.md.

This ability for multiple database backends has been available for a long,
long time.  Looks like this was simply not updated when the interface was
added.