add link to structopt-derive documentation from structopt crate
//! `StructOpt` trait definition
//!
//! This crate defines the `StructOpt` trait. Alone, this crate is of
-//! little interest. See the `structopt-derive` crate to
-//! automatically generate implementation of this trait.
+//! little interest. See the
+//! [`structopt-derive`](https://docs.rs/structopt-derive) crate to
+//! automatically generate an implementation of this trait.
extern crate clap as _clap;
