Update README file for src/ directory
Doc: grammar fix in intro.md
auto merge of #14621 : Sawyer47/rust/fix-readme, r=alexcrichton
Implement ToJson for &[T], and add tests. Closes #14619
auto merge of #14622 : reillywatson/rust/master, r=alexcrichton
auto merge of #14625 : japaric/rust/slice-tojson, r=alexcrichton

Let me know if the amount of tests is enough or too much.