[PORT] Update stale link

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3275

Fixes #3271

The main README has:

# Click here to find out what's new with Bot Framework which points to an archived Repo.

It was updated for Ignite in November. If we're still going to use it, feel free to disregard this PR.

I also considered linking to the Releases page of this repo, but felt it was better to just remove the link.