docs: Link to clap3 migration guide

clap-rs/clap#3422 drew attention to the lack of discoverability of the
migration guide.  We've added it to the clap FAQ item about structopt.
Adding it here as well in hopes people will find it.

README.md

@@ -11,6 +11,8 @@ As clap v3 is now out, and the structopt features are integrated into (almost as
 
 Bugs will be fixed, and documentation improvements will be accepted.
 
+See the [structopt -> clap migration guide](https://github.com/clap-rs/clap/blob/master/CHANGELOG.md#migrate-structopt)
+
 ## Documentation
 
 Find it on [Docs.rs](https://docs.rs/structopt).  You can also check the [examples](https://github.com/TeXitoi/structopt/tree/master/examples) and the [changelog](https://github.com/TeXitoi/structopt/blob/master/CHANGELOG.md).