Add some readme doc

frame/contracts/README.md

@@ -135,6 +135,18 @@ to `error` in order to prevent them from spamming the console.
 `--dev`: Use a dev chain spec
 `--tmp`: Use temporary storage for chain data (the chain state is deleted on exit)
 
+## Host function tracing
+
+For contract authors it can be a helpful debugging tool to see which host functions are called with 
+which arguments and what was the result. In order to see these messages on the node console,
+the log level for the `runtime::contracts::strace` target needs to be raised to the `trace` level 
+
+Example: 
+
+```bash
+cargo run --release -- --dev -lerror,runtime::contracts::strace=trace,runtime::contracts=debug
+```
+
 ## Unstable Interfaces
 
 Driven by the desire to have an iterative approach in developing new contract interfaces

frame/contracts/proc-macro/src/lib.rs

@@ -607,9 +607,8 @@ fn expand_functions(def: &EnvDef, expand_blocks: bool, host_state: TokenStream2)
 		let is_stable = f.is_stable;
 		let not_deprecated = f.not_deprecated;
 
-		// wrapped host function body call with traces when target is`runtime::contracts::strace` and level is `trace`
-        // e.g ./target/release/substrate-contracts-node -l "fatal,runtime::contracts::strace=trace" --dev
-		//WIP(pg): replace comment above with link once the doc has been added to the readme
+		// wrapped host function body call with host function traces
+		// see https://github.com/paritytech/substrate/tree/master/frame/contracts#host-function-tracing
 		let wrapped_body_with_trace = {
 			let params_fmt_str = params.clone().filter_map(|arg| match arg {
 				syn::FnArg::Receiver(_) => None,