Add dedicated markdown docs for ApproximateEquality

[Markus Winter]

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

[Stephen Canon]

Updated title; we have a sketch of header doc already (which can be improved), but what I really want an issue for is a markdown doc that gives a general introduction and lots of examples.