Add TODO item to remove the panic handler

Author: dmitry-markin

runtime/src/panic_handler.rs

@@ -1,5 +1,8 @@
 //! Panic handler for builds without `std`.
 
+// TODO: Remove this file once `sp-io` is uipgraded.
+//       See https://github.com/paritytech/polkadot-bulletin-chain/issues/18.
+
 use sp_core::LogLevel;
 
 #[cfg(not(feature = "std"))]