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/error_handling/README.md

@@ -0,0 +1,5 @@
+For this exercise check out the chapters:
+- [Error Handling](https://doc.rust-lang.org/book/second-edition/ch09-02-recoverable-errors-with-result.html) 
+- [Generics](https://doc.rust-lang.org/book/second-edition/ch10-01-syntax.html) 
+
+of the Rust Book.

exercises/functions/REAMDE.md

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+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.

exercises/if/REAMDE.md

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+For this exercise check out the chapter [If](https://doc.rust-lang.org/book/second-edition/ch03-05-control-flow.html) of the Rust Book.

exercises/macros/README.md

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+For this exercise check out the section [Macros](https://doc.rust-lang.org/book/first-edition/macros.html) and the chapter
+[Macros Appendix](https://doc.rust-lang.org/book/2018-edition/appendix-04-macros.html) of the Rust Book and [The Little Book of Rust Macros](https://danielkeep.github.io/tlborm/book/index.html). 

exercises/modules/README.md

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+For this exercise check out the [Modules](https://doc.rust-lang.org/book/second-edition/ch07-01-mod-and-the-filesystem.html) chapter of the Rust Book.

exercises/move_semantics/README.md

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+These exercises are adapted from [pnkfelix](https://github.com/rustlings/rustlings/blob/master)'s [Rust Tutorial](https://pnkfelix.github.io/rust-examples-icfp2014/) -- Thank you Felix!!!
+
+For this exercise check out the chapters:
+- [Ownership](https://doc.rust-lang.org/book/second-edition/ch04-01-what-is-ownership.html)
+- [Reference and borrowing](https://doc.rust-lang.org/book/second-edition/ch04-02-references-and-borrowing.ht)
+
+of the Rust Book.

exercises/standard_library_types/README.md

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+For this exercise check out the chapter [Shared-State Concurrency](https://doc.rust-lang.org/book/second-edition/ch16-03-shared-state.html) of the Rust Book.

exercises/tests/README.md

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+Going out of order from the book to cover tests -- many of the following exercises will ask you to make tests pass!
+
+For this exercise check out the section [How to Write Tests](https://doc.rust-lang.org/book/second-edition/ch11-01-writing-tests.html) of the Rust Book.

exercises/threads/README.md

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+For this exercise check out the [Dining Philosophers example](https://doc.rust-lang.org/1.4.0/book/dining-philosophers.html) and the chapter [Concurrency](https://doc.rust-lang.org/book/second-edition/ch16-01-threads.html) of the Rust Book.