Add description for book.toml

`book.description` value in book.toml provides nice HTML document metadata. 

Ref: https://github.com/rust-lang/mdBook/blob/8e673c9/src/theme/index.hbs#L12

Author: weihanglo

book.toml

@@ -1,6 +1,6 @@
 [book]
 title = "Rust By Example"
-description = "A description"
+description = "Rust by Example (RBE) is a collection of runnable examples that illustrate various Rust concepts and standard libraries."
 author = "The Rust Community"
 
 [output.html.playpen]