update readme
Closes #4

LGTM given by: @frankhock

Squashed commit of the following:

commit cf5d94eaf774cbddce0fc69288ddf76360db1a53
Author: Frank Hock <frank@thinkmechanic.com>
Date:   Tue Nov 4 13:36:36 2014 -0500

    update readme

commit 2b4e7b5f93d069050b04b1c284091bf1e7b72502
Author: Frank Hock <frank@thinkmechanic.com>
Date:   Tue Nov 4 13:24:08 2014 -0500

    working slug percentage

commit 5af89718f183d50b3b120bbb52a0932a9fa9d747
Author: Frank Hock <frank@thinkmechanic.com>
Date:   Tue Nov 4 09:02:22 2014 -0500

    most_improved_batting_average working