Change log and acknowledgements for WCAG 3

As part of publishing WCAG 3 and the Explainer, I had a note to check 
the changelog and acknowledgements. I created a changelog for the 
guidelines:

https://raw.githack.com/w3c/silver/2021-11_WD/guidelines/index.html#change-log

Let me know if you have comments on structure or content. This is only 
in the the publication branch at the moment, but should go into main 
when ready.

For acknowledgements, I just put the same ones into the Explainer as 
were in the Guidelines. Those are at:

https://raw.githack.com/w3c/silver/2021-11_WD/guidelines/index.html#acknowledgements

Let me know if I should do something different.

Publication is queued for Tuesday, I can make last-minute changes (non 
time-consuming ones) through Monday.

Michael