Explicit description of hash-to-group

[hdevalence]

Add an explicit description of how to use Elligator to provide hash-to-group. Document behaviour around the edges.

Depends on the description of Elligator in #15

[hdevalence]

This is different from #29: while #29 describes a map from field elements to the group, this describes how to map uniformly-distributed bytes to a uniformly-distributed group element.