examples-features/ctap.rs: Add a warning at the top of the file

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

examples-features/ctap.rs

diff --git a/examples-features/ctap.rs b/examples-features/ctap.rs
index c037685..3a66eff 100644
--- a/examples-features/ctap.rs
+++ b/examples-features/ctap.rs
@@ -1,3 +1,6 @@
+//! This is a featured CTAP example
+//! WARNING! This currently uses unsound crypto operations
+//! This is only a demo and should not be used in real enviroments
 #![no_std]
 #![feature(alloc_error_handler)]