Add flutter_map_marker_popup to plugins in README.md

[rorystephenson]

I noticed there are a lot of requests for how to add popups and having tried the suggested approaches myself I found there were some drawbacks including:

• The popup disappears once the marker disappears even though the popup is big enough to still be visible.
• The popup is not tappable.
• A fixed size was required for the popup.
• It was not easy to choose how it is positioned.

So I wrote my own code to handle this which I've since moved to a plugin:

https://github.com/rorystephenson/flutter_map_marker_popup