rust-guide::20 method syntax - Suggestion to explain method chaining and types of self

[anmej]

cc @steveklabnik

In chapter 20 of the guide you write that x.foo().bar().baz() has better readability than baz(bar(foo(x))). However, afterwards you only give an example of a simple fn area(&self) -> f64 method.

I suggest adding an example of a method that allows chaining. Something like fn foo(&mut self) -> &mut T {...}, where T is the type of the struct.

In addition, it would be nice to explain the distinction between methods that take 'self', 'mut self', '&self' and '&mut self'. All of those can be used to implement method chaining, but with different purposes - some can be used to mutate the original value, while others can't, etc...