Update README to point to the extra plugin

Author: hansthen

README.rst

@@ -82,3 +82,4 @@ Plugins:
 - https://github.com/onaci/folium-glify-layer: provide fast webgl rendering for large GeoJSON FeatureCollections
 - https://github.com/carlosign/Folium.ControlCredits-Plugin: displaying credits in the corner. Display an image/logo, clicking it will expand to show a brief message with credits and links.
 - https://github.com/JohnyCarrot/folium-geocoder-own-locations: a geocoder that accepts a list of suggestions at creation time.
+- https://github.com/iwpnd/folium-vectortilelayer: a tile layer that zooms and stretches beyond the maximum and minimum of the tile provider