Acknowledgements added to Silver draft

We realized this week that we never added the acknowledgements section 
to the WCAG 3.0 guidelines draft after porting the base of the current 
version from a Google doc. We've fixed this:

https://w3c.github.io/silver/guidelines/#acknowledgements

I believe this is an editorial change that doesn't impact the open Call 
for Consensus to publish. Please check and let us know if any names seem 
missing.

Michael