Please check WCAG acknowledgements

I've taken pass at updating the acknowledgements for WCAG 2.1:

https://w3c.github.io/wcag21/guidelines/#acknowledgments

I'd appreciate reviews of it. The first list, "Participants of the WCAG 
WG active in the development of this document", lists anybody who has 
ever contributed to the development of WCAG 2.1 through its lifecycle, 
including via task forces. If there is somebody who you remember has 
contributed but is missing, please let me know. If there is a name there 
whom you believe never contributed, let me know as well, though keep in 
mind many people have contributed via task forces and might not be on 
the radar of the larger group.

Michael