Update ReadMe.md

[casella]

Added a short explanation in the Roadmap on how to actually read the proposal, which is not obvious at first sight.

[henrikt-ma]

Perhaps a better place to put the new text is under Summary, right before In a sentence (or two)?

If placed there, it would also be good to recommend reading the rest of the Summary and at least skimming the roadmap items before proceeding to the other documents.