Created sub READMEs for CLI only version

Sub READMEs contain the relevant resources from the main
README of the web version. All second edition Rust Book links
were updated to the 2018 edition.

Fixed to typos in main README.md.

exercises/functions/REAMDE.md

@@ -0,0 +1 @@
+For this exercise check out the chapter [Functions](https://doc.rust-lang.org/book/second-edition/ch03-03-how-functions-work.html) of the Rust Book.