Add a version matrix in `README.md` to show which Web Chat support which version of Adaptive Card

[compulim]

Based on a supportability work item filed at https://fuselabs.visualstudio.com/BotFramework/_workitems/edit/42683.

We have the Adaptive Card version information in CHANGELOG.md. Eyal suggested to surface that information as a table in our README.md, or somewhere easy to find.

[Bug]