Update README.mkd

I think it would be a good idea to demonstrate usage of `hiera_hash()` (rather than `hiera()`) which is very useful when you are using multiple hiera YAMLs.

For example, in many organizations you may have a `common.yaml` that includes all sysadmin users.  And another `environment/dev.yaml` that includes additional users from the developer team. Using `hiera_hash()` would combine both YAML's.
Merge pull request #15 from eshkay/patch-1

Update README.mkd to suggest hiera_hash() over hiera()