Renamed readme files properly

Author: Jean-Philippe Moresmau

exercises/functions/README.md

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+### 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)

exercises/strings/README.md

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+### Strings
+
+Rust has two string types, a string slice (`&str`) and an owned string (`String`).
+We're not going to dictate when you should use which one, but we'll show you how
+to identify and create them, as well as use them.
+
+#### Book Sections
+
+- [Strings](https://doc.rust-lang.org/stable/book/ch08-02-strings.html)