docs: update readme for contract fixtures

tests/xcm-simulator/src/README.md

@@ -19,4 +19,8 @@ It's possible that in the future we decide to define a different runtime type (p
 
 Running tests is same as with any other unit tests:
 
-`cargo test -p xcm-simulator-tests`
+`cargo test -p xcm-simulator-tests`
+
+# ink! Contract fixtures
+All the contract used in XCM Scenarios should be placed in [contracts](../contracts/) dir. The [cargo build script](../build.rs) will pick them and build them before executing tests automatically.
+Please refer the env variables used in build script to configure the contract building - [`build.rs`](../build.rs).