Add explanation of operators in the example.

[nmn]

I found it super hard to read the example without knowing what the operators meant. So when I figured it out I add an explanation to the readme. This way, I also get to verify my understanding.

[seliopou]

Hey thanks for this pull request. I think the motivation behind it is good, but it also kindof duplicates some of the documentation that's already in the .mli. Just having a reference to the mli for folks that need further explanation may be less duplicative and ensure that documentation remains consistent. Just something like "For an explanation of the operators involved in the implementation of this parser, see the (documentation in the ml)".

If you're interested in making that change, I'll certainly merge it. Otherwise, let me know and I'll go ahead and make the change.

[seliopou]

Closing in favor of #158. Thanks for pointing this out!