Add cargo-modules. Closes mainmatter#101

Author: LukeMathWalker

book/src/03_ticket_v1/03_modules.md

@@ -112,3 +112,10 @@ where each name comes from and potentially introducing name conflicts.\
 Nonetheless, it can be useful in some cases, like when writing unit tests. You might have noticed
 that most of our test modules start with a `use super::*;` statement to bring all the items from the parent module
 (the one being tested) into scope.
+
+## Visualizing the module tree
+
+If you're struggling to picture the module tree of your project, you can try using 
+[`cargo-modules`](https://crates.io/crates/cargo-modules) to visualize it!
+
+Refer to their documentation for installation instructions and usage examples.