Describe key optionality property of maps on the README.

[jan-dubsky]

Closes #13

[Hrubon]

Nice elaboration. But I am not sure whether it is desirable to have such elaboration present on "homepage" of the project. I would be for keeping the homepage as concise and readable as possible. This nicely explains the motivation and design choices, but it goes into detail just too much, in my opinion.

I would suggest putting this to the end of the GitHub issue as a conclusion. Or at least put it on a separate README page (not sure whether and how actually GitHub supports that). And write a short paragraph at the end of the Parsing maps section on homepage, stating just something like: "Beware of maps, they are implicitly optional. See this GitHub issue (or this page) for more details, reasons, and discussion."

[Hrubon]

Approved. I have also added a headline that will emphasize the short-summary in section about maps.