Dead link in README.md

The link pointing to "LLNL co-design pages" in the Introduction section of README.md is a dead link. Perhaps it can be refreshed to this one instead? https://asc.llnl.gov/codes/proxy-apps/quicksilver

(Also, I don't know who in ECP I should contact for this one, but the same dead link is listed in https://proxyapps.exascaleproject.org/app/quicksilver/ )

Start in the Introduction section of README.md and inspect the link labeled "LLNL co-design pages." Verify the proposed https://asc.llnl.gov/codes/proxy-apps/quicksilver destination, then update the README link and check the related listing at https://proxyapps.exascaleproject.org/app/quicksilver/ to determine whether it is within this issue's scope.

Written by the indexing model from the issue text.