Put a simple example of use in the Readme

[JulianBirch]

The detailed documentation doesn't really start you off on how to use it.

[frenchy64]

Until we address this, the tests can help start you off.

https://github.com/pallet/stevedore/blob/develop/test/pallet/stevedore/bash_test.clj#L51

[JulianBirch]

Thanks. Even just putting a link to the relevant tests in the readme would be an improvement over the current "I just found this on github" experience.