Use one-to-one mapping example in README

Most users will probably have one bucket type that holds all bucket &
data. Use this style (one-to-one mapping) as the README example rather
than associating the index at the type level which causes it to be
used for all buckets (one-to-many).
Merge pull request #244 from basho/docs/rz/readme-btype

Use one-to-one mapping example in README