Add dedicated markdown docs for ApproximateEquality

Write documentation for Approximate Equality and link document to README.md

See:

> We should definitely link to detail on ApproximateEquality, but we should link to a markdown document, rather than the implementation. Let's remove this for now and open an issue to track adding one.

Originally posted by @stephentyrone in #150

Start by locating the ApproximateEquality implementation and the README.md link area. Add a dedicated Markdown document explaining Approximate Equality, then link that document from README.md and verify the links and documentation render correctly.

Written by the indexing model from the issue text.