Example myapp-03 codegen requires libclang, please add to documentation

[FallingSnow]

I spent some time trying to track down the cargo xtask codegen error I was running into while trying to run example myapp-03. The error is somewhat misleading with the message No such file or directory..

I was able to debug this by using the bindgen-cli to try to complete the same task which resulted in a libclang missing error. After installing clang the example codegen completed successfully.

I suggest adding libclang as a dependency in the readme.