Add note

README.md

@@ -93,6 +93,8 @@ It is now early August, and *Easy Rust* is almost 300 pages long. I am still wri
 - [Crates and modules](#crates-and-modules)
 - [Testing](#testing)
   - [Test-driven development](#test-driven-development)
+- [External crates](#external-crates)
+  - [Rand](#rand)
 
 ## Rust Playground
 
@@ -9877,6 +9879,8 @@ For this section you *almost* need to install Rust, but we can still use just th
 
 That means that it is normal in Rust to bring in an external crate for a lot of basic functions. The idea is that if it is easy to use external crates, then you can choose the best one. Maybe one person will make a crate for one function, and then someone else will make a better one.
 
+In this book we will only look at the most popular crates, the crates that everyone who uses Rust knows.
+
 To begin learning external crates, we will start with the most common one: `rand`.
 
 ### rand