Add Rustlings

Even though Rustlings isn't a book it needs an
honourable mention. But non-book titles are
going to be very much the exception!

src/official.md

@@ -5,6 +5,7 @@ Many of them are mentioned in [Learn Rust](https://www.rust-lang.org/learn).
 
 Introductory:
 * [Rust by Example](https://doc.rust-lang.org/stable/rust-by-example/) - runnable examples illustrating Rust concepts
+* [_Rustlings_](https://github.com/rust-lang/rustlings) - small exercises to get you used to reading and writing Rust code
 * [The Rust Programming Language](https://doc.rust-lang.org/book/) - "the book"
 
 Core:

src/title-page.md

@@ -7,6 +7,6 @@ This book is a treasure-trove of Rust books:
 
 If you know of others, why not submit a [pull request](https://github.com/lborb/book#pull-requests)?
 
-The titles of books not in
+Titles not in
 [mdbook](https://github.com/rust-lang/mdBook) format are _italicised_.
 For more books in other formats, see [Rust Books](https://github.com/sger/RustBooks).