docs: clarify applet availability and server implementation status

Update README to specify that irtt-tui requires the tui feature and explicitly state that server support is not yet implemented.

crates/irtt-cli/src/cmd/mod.rs

@@ -3,5 +3,4 @@ pub mod client;
 #[cfg(feature = "tui")]
 pub mod tui;
 
-// Future command applets, such as a server applet, belong here behind their
-// own feature gate.
+// Additional command applets belong here behind their own feature gate.