Add deprecation comment

tcharding · tcharding · commit dbb3edd482b4 · 2021-11-16T13:01:51.000+11:00
Module `contracthash` is deprecated, add this info to the module
rustdoc.
diff --git a/src/util/contracthash.rs b/src/util/contracthash.rs
@@ -17,6 +17,7 @@
 //! See Appendix A of the Blockstream sidechains whitepaper at
 //! <http://blockstream.com/sidechains.pdf> for details of what this does.
 //!
+//! This module is deprecated.
 
 #![cfg_attr(not(test), deprecated)]
 
