add the text "incremental quantile estimation" to the README so that this library is discoverable via that name

[jrwren]

I'm skimming through Numerical Recipes and IQAgent (8.5.2 - Single-Pass Estimation of Arbitrary Quantiles) caught my eye. After searching a bit I found various names for this. When I searched pkg.go.dev for "incremental quantile estimation" this module did not show as a result. IMO it should.

Would you accept a README update PR with some additional description? I guess in some sense this is SEO.