redo the section readmes

exercises/functions/REAMDE.md

@@ -1 +1,7 @@
-For this exercise check out the chapter [Functions](https://doc.rust-lang.org/book/2018-edition/ch03-03-how-functions-work.html) of the Rust Book.
+### Functions
+
+Here, you'll learn how to write functions and how Rust's compiler can trace things way back.
+
+#### Book Sections
+
+- [How Functions Work](https://doc.rust-lang.org/stable/book/ch03-03-how-functions-work.html)