Guide: strings

I decided to change it up a little today and hack out the beginning of the String guide. Strings are different enough in Rust that I think they deserve a specific guide, especially for those who are used to managed languages.

I decided to start with Strings because they get asked about a lot in IRC, and also based on discussions like this one on reddit: http://www.reddit.com/r/rust/comments/2ac390/generic_string_literals/

I blatantly stole bits from our other documentation on Strings. It's a little sparse at current, but I wanted to start somewhere.

I am not exactly sure what should go in "Best Practices," and would like the feedback from the team on this. Specifically due to comments like this one: http://www.reddit.com/r/rust/comments/2ac390/generic_string_literals/citmxb5
> you cannot construct a value of type &mut str (without unsafe)

[You can][code]. ;P

(Of course, you can't mutate it.)

[code]: http://play.rust-lang.org/?run=1&code=fn%20main%28%29%20{%0A%20%20%20%20let%20x%3A%20%26mut%20str%20%3D%20%22a%22%3B%0A%20%20%20%20println!%28%22{}%22%2C%20x%29%3B%0A}