Update readme

Mention setting up a sample database configuration file so that `rake
db:migrate` works.

Also note that this project *does* work under Ruby 1.9.x.

Change-Id: I07596a17d251f4b13cc75f86298e79c4a8fce43c
Reviewed-on: https://gerrit.causes.com/22707
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>