Add instructions and error messages about generate-native-simulator (#21)

Author: joii2020

README.md

@@ -238,3 +238,20 @@ $ make clippy
 The template is tailored built for usage outside of workspace, typically, it is not expected to be used inside a workspace. Feel free to compare it with the default `contract` workspace for differences.
 
 This standalone template also has its own test setup, where in a workspace, a dedicated `tests` crate will handle most of the testing work.
+
+### Native Simulator debug
+
+The `generate-native-simulator` command in the `Makefile` generates a native simulator. It **requires** the `CRATE` parameter to specify an existing subproject. If the parameter is missing or invalid, the command will fail.
+
+```bash
+make generate-native-simulator CRATE=<subproject_name>
+```
+
+To generate a simulator for the `example_crate` subproject:
+
+```bash
+make generate-native-simulator CRATE=example_crate
+```
+
+- The `CRATE` parameter must refer to a subproject.
+- Missing subprojects will cause the command to fail.

workspace/Makefile

@@ -127,12 +127,20 @@ generate:
 
 generate-native-simulator:
 	@set -eu; \
+	if [ -z "$(CRATE)" ]; then \
+		echo "Error: Must have CRATE=<Contract Name>"; \
+		exit 1; \
+	fi; \
 	mkdir -p native-simulators; \
 	cargo generate $(TEMPLATE_TYPE) $(TEMPLATE_REPO) native-simulator \
 		-n $(CRATE)-sim \
 		--destination native-simulators; \
 	sed '/@@INSERTION_POINT@@/s/$$/\n  "native-simulators\/$(CRATE)-sim",/' Cargo.toml > Cargo.toml.new; \
-	mv Cargo.toml.new Cargo.toml;
+	mv Cargo.toml.new Cargo.toml; \
+	if [ ! -f "contracts/$(CRATE)/Cargo.toml" ]; then \
+		echo "Warning: This is a non-existent contract and needs to be processed manually"; \
+		echo "		Otherwise compilation may fail."; \
+	fi;
 
 prepare:
 	rustup target add riscv64imac-unknown-none-elf