Update readme to specify virtual threads requirement

Clarify that virtual threads aren't just a default or suggestion,
but a hard requirement.

Related-to: avaje#320 (comment)
Signed-off-by: Mahied Maruf <contact@mechite.com>

Author: mechite

README.md

@@ -19,7 +19,7 @@ Features:
 - Json SPI
 - File Uploads
 - SSL/mTLS configuration
-- Virtual threads enabled by default
+- Virtual threads enabled by default and required
 - Multi-Server with any implementation of `jdk.httpserver` (Jetty, Robaho, built-in, etc)
 
 ## Quick Start