README: Add legend for Features section

As suggested by David: those emojis might not be entirely obvious.

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

README.md

@@ -247,6 +247,9 @@ speeding up local connections, and usually requiring NAT. _pasta_:
 
 ## Features
 
+✅: done/supported, ❌: out of scope, 🛠: in progress/being considered
+⌚: nice-to-have, eventually
+
 ### Protocols
 * ✅ IPv4
     * ✅ all features, except for