The documentation in the README was a bit out of date when it comes to

kgb 2.0, talking about the necessity of using the .spy attribute for

public functions and not including all supported Python versions. It was

also in Markdown, meaning it wasn't helpful at all for PyPI.
This change moves the README to ReST, updates the contents to be more

correct with kgb 2.0, and attaches it to the package metadata.

Rendered the README and read through it. Didn't spot any spelling errors

or other issues.