Update 3.1-frontend.md

src/3.1-frontend.md

@@ -3,3 +3,10 @@ Traditionally, performing code analysis requires modifying the compiler source c
 Developers then need to recompile the compiler to activate these new passes, which can be cumbersome. 
 The Rust compiler offers a more portable way to conduct code analysis using the [rustc_driver](https://rustc-dev-guide.rust-lang.org/rustc-driver.html).
 We refer to this approach as the **frontend** method because it allows developers to directly access internal compiler data and perform analysis as callbacks.
+
+## Custom Cargo Commands
+To support project-level program analysis, we want the analysis tool to be integrated into cargo as [subcommands](https://doc.rust-lang.org/cargo/reference/external-tools.html). To this end, we can name the tool as cargo-toolname and place it in $CARGO_HOME/bin or $PATH. Then we can execute the tool via the following command.
+```
+cargo toolname -more_arguments
+```
+Cargo will automatically search the binaries named cargo-toolname from the paths.