Better docs

[dhardy]

This crate looks nice but before attempting to use it I'd like to see better documentation.

From what I can tell:

• i18n! must be called from the crate root; it generates items which are expected to be found in the crate root
• t! returns a String
• t!("foo", x = 1) is equivalent to t!("foo", x => 1) (why support both?)
• t!("foo", locale = "en") — parameter locale has special handling
• Unused parameters like t!("foo", unused_param=5) are allowed and ignored
• Passing tuples or structs as parameters is not supported
• Every passed parameter must support std::fmt::Display (even if unused)