Add a list of examples of MapML files or URLs to the readme.md

[joanma747]

Hi,

Each time that I have to look for an example I have a hard time until I get one that is representative of what I need. I propose to add a list of examples to this github area.

[prushforth]

I believe that is a good idea. Also, you can use the "View source for top map layer" link on all of these MapML examples to see what is useful map markup. There are not too many simple vector examples yet, though.