Link to CHANGES from README

And use absolute URLs for GitHub links so they work from PyPI.

[CI skip]
Remove 2,505-line changelog from description

It made the PyPI listing page very long and requires a lot of scrolling to get down to the files, annoying for downstream packagers.

Instead it's linked from the README. 

[CI skip]
Merge pull request #854 from hugovk/changelog

Link to 2,505-line changelog instead of showing it all on PyPI