Outdated link to mdBook in README, CONTRIBUTING

[mikong]

The mdbook link points to https://github.com/azerupi/mdBook which should now be https://github.com/rust-lang-nursery/mdBook.

There's also a couple of links to mdbook support that point to https://github.com/azerupi/mdBook/issues/5 which should now be rust-lang/mdBook#5.

PR coming soon...