Add trait recommendations.

book/src/04_traits/14_outro.md

@@ -4,6 +4,25 @@ We've covered quite a few different traits in this chapter—and we've only scra
 It may feel like you have a lot to remember, but don't worry: you'll bump into these traits
 so often when writing Rust code that they'll soon become second nature.
 
+## Closing thoughts
+
+Traits are powerful, but don't overuse them.  
+A few guidelines to keep in mind:
+
+- Don't make a function generic if it is always invoked with a single type. It introduces indirection in your 
+  codebase, making it harder to understand and maintain. 
+- Don't create a trait if you only have one implementation. It's a sign that the trait is not needed.
+- Implement standard traits for your types (`Debug`, `PartialEq`, etc.) whenever it makes sense.
+  It will make your types more idiomatic and easier to work with, unlocking a lot of functionality provided 
+  by the standard library and ecosystem crates.
+- Implement traits from third-party crates if you need the functionality they unlock within their ecosystem. 
+- Beware of making code generic solely to use mocks in your tests. The maintainability cost of this approach 
+  can be high, and it's often better to use a different testing strategy. Check out the 
+  [testing masterclass](https://github.com/mainmatter/rust-advanced-testing-workshop) 
+  for details on high-fidelity testing.
+
+## Testing your knowledge
+
 Before moving on, let's go through one last exercise to consolidate what we've learned. 
 You'll have minimal guidance this time—just the exercise description and the tests to guide you.