Clarify this repo applies to original data square

[rootulp]

Context

There is some ambiguity in the README. One could interpret "data square" operations to deal with rsmt2d. This repo contains types and utilities to create the original data square rather than the extended data square.

Proposal

Clarify that this repo has data types (shares, namespaces) to construct the original data square. Diagrams may help here: #17. Perhaps also describe the motivation for extracting this from celestia-app (so that consumers of these Go modules don't have to pull in transitive dependencies like Cosmos SDK)