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
`"Hello".to_string()` is _definitely_ the current convention.

FWIW, there's a fair amount of general grumbling about the ergonomics of String/str right now, and the fact that you have to do this for literals at all. So hopefully we'll alleviate the need for any conversion later on. But for now, stick with `.to_string()`.