Add blurb at top of README that points to core Ape repo

[antazoey]

Overview

If you just stumble upon an ape plugin repo, it does not make a lot of sense. Especially one named ape-ledger, that almost sounds like a whole blockchain itself or something.

Perhaps we should have a big blurb at the top of each README saying "YO, THIS IS A PLUGIN FOR THIS OTHER PYTHON PACKAGE".

Specification

Something like

This package is a plugin for the [eth](link) python Ethereum framework. To use this plugin, you have to have ape installed. please and thank you.

Dependencies

Include links to any open issues that must be resolved before this feature can be implemented.