Add example code

[dtolnay]

The readme explains in words what to do, but it would be good to show a code snippet using momo. In fact most of what's in https://llogiq.github.io/2019/05/18/momo.html would be well suited to the readme.

[llogiq]

There's now an example that shows usage on both functions and methods.

[benma]

I read the example and the README and didn't get a clear understanding until I read the first example in the above link. The above should ideally be in the README to get a quick understanding of what is happening.