Add Features and Perspectives to Readme

Synthetic view of the current state of the library and the planed short term improvements.
I fully understand that you are proud of the features you are working on. A readme is really just meant for a quick orientation of the user, so try to keep information about ongoing development as brief as possible. So instead of having a "Perspectives" heading (which sounds a bit weird in English), I'd rather just add a paragraph after the bullet points in the "Features" section. Maybe something like

"Stay tuned for more exciting features like clamping 2D geometries to the terrain, animated transitions from map to globe view, and synchronization of maps in projections other than EPSG:4326 and EPSG:3857".