unicorn 5.8.0 - rack.after_reply support

This release supports env['rack.after_reply'] which allows
rack middleware to pass lambdas to be executed after the client
connection is closed, matching functionality in Puma.

Thanks to Blake Williams for this patch:
https://yhbt.net/unicorn-public/9873E53C-04D3-4759-9678-CA17DBAEF7B7@blakewilliams.me/

The top-level of our website is now simpler and no longer
redundant with the contents of https://yhbt.net/unicorn/README.html
(which contains the old content)