Focus the README on end user usage

[demeringo]

Problem

The README describe current implementation details but does not provide much guidance to end-users (accessing or contributing to the data).

Solution

Extract the technical documentation to a separate file that is linked from the README.

Update the README to focus on the objectives of the project and more importantly how to contribute to the data (using the widget or by directly openning a PR).

Alternatives

Additional context or elements