[Rust] Recommend style based on 'rustfmt' defaults (#7335)

Rustfmt is widely accepted in the rust community. The actual style it
enforces is not completely stable, but running a reasonably recent
rustfmt and reading the short rfc-based style guide seems better than
nothing by a long shot.

Author: euank

CONTRIBUTING.md

@@ -58,6 +58,7 @@ Code change should conform to the programming style guide of the respective lang
 - Python: https://www.python.org/dev/peps/pep-0008/
 - R: https://google.github.io/styleguide/Rguide.xml
 - Ruby: https://github.com/bbatsov/ruby-style-guide
+- Rust: https://github.com/rust-lang-nursery/fmt-rfcs/blob/master/guide/guide.md (the default [rustfmt](https://github.com/rust-lang-nursery/rustfmt) configuration)
 - Scala: http://docs.scala-lang.org/style/
 - Swift: [Apple Developer](https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html)
 - TypeScript: https://github.com/Microsoft/TypeScript/wiki/Coding-guidelines