Document WASM/playground maintenance mode

[Xophmeister]

The WASM target and web-based playground for Topiary are currently in maintenance mode and not actively developed. Indeed, no changes are built or deployed from main; we have a special playground branch, which has diverged significantly from main, for this.

This fact should be clearly documented in the README or contributor's guidance, to:

1. Manage expectations
2. Prevent unnecessary work from future contributors

[aspiwack]

This makes me sad though. Is there no hope to revive the playground?

[Xophmeister]

Likewise. The playground is still online, but it hasn't been updated for some time. It's definitely on the backlog to return it to its former glory (and then some) 🤞