Update README.md
Squashed commit of the following:

commit a58847f38d750fe711c97ebbdc19b2a3eea47b8a
Merge: b84e62d 06c7355
Author: Mattt Thompson <m@mattt.me>
Date:   Thu Mar 6 10:01:50 2014 -0800

    Merge branch 'master' of github.com:cbx/rack-passbook into cbx-master

commit 06c735525e722b3e57f8c9921ee02084353fa4dc
Author: Clemens Beer <clemens.beer@gmail.com>
Date:   Thu Jan 16 15:55:11 2014 +0100

    move VERSION back to gemspec

    module Rack::Passbook clashed with class Rack::Passbook < Sinatra::Base in Ruby 2.1

commit 5fe17bce2b0c49f3b379b9231d7c703e85fc45fc
Author: Clemens Beer <clemens.beer@gmail.com>
Date:   Wed Dec 11 17:29:09 2013 +0100

    move version to separate file

commit 642a0f10494092bce8fbc34e5c44892608ea510f
Author: Clemens Beer <clemens.beer@gmail.com>
Date:   Wed Aug 14 23:17:51 2013 +0200

    use correct table name & canonicalize timestamp in query

commit ff269b004730038871d9ffc4ee61f37d8115bfea
Author: Clemens Beer <clemens@tupalo.com>
Date:   Tue Aug 13 19:41:38 2013 +0200

    Fix push_token length validation

Signed-off-by: Mattt Thompson <m@mattt.me>
Bumping version to 0.2.0