Release notes for 1.0a20

[simonw]

[simonw]

I had Codex Cloud take a first stab at this: https://gist.github.com/simonw/b23c2d8ab32e944ef59dfbef83e7663f

Write the changes for 1.0a20 - based on a diff from main to 1.0a19 - and add that to changelog.rst

Imitate the change notes for the larger releases in that file

Highlight the permissions system changes in the opening paragraph