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 like calling them views, I think it makes the behaviour of the slice types clearer, since they're just pointers into someone else's memory (i.e. a view of it). This matches other languages/libraries, e.g. Numpy's [view](http://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.view.html) type.

When people on IRC ask about `&str` vs. `String` and `&[T]` vs. `Vec`, they seem to get the hang of it (or at least, say they do) when describing the slices as "views".