Defining the Hydra Way for Curate

The Hydra Stack has a set of promises that we should call greater
attention to. This is especially important for new developers.

We have an anti-harassment policy that has been defined and ratified.

We have a set of community principles that we should call attention
to.

Also removing Curate references from CONTRIBUTING.md in preparation
for community adoption of the Contributing document into Hydra's core
gems.

Related to projecthydra/hydra#34
Related to projecthydra/hydra#35
Related to projecthydra/hydra#36
[skip ci]
Changing the sort fields to just have one stored_sortable Solr field. The previous code was just creating one solr field which was multi-valued and not sortable. Not sure if that's a configuration issue in Curate or a Solrizer bug