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
I'm still not happy with the phrasing here, "it has the usual associated lifetime". The goal of this sentence is to convey the idea that the lifetime can be omitted in a lot of cases, right? Omitting the lifetime is of course not limited to just function arguments, you can also say `let x: &str = "foo"`.

Is there some other guide that uses a similar phrasing here that you're trying to reference? If not, I'd suggest perhaps something like

> The `&str` type can be written without an explicit lifetime in many cases, such as in function arguments. In these cases the lifetime will be inferred:

although I feel like it would be good to explicitly correlate this with the inferred lifetimes of arbitrary `&T` refs.