Update README.md

Author: mourner

README.md

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 <img src="http://leafletjs.com/docs/images/logo.png" alt="Leaflet" />
 
+ATTENTION: the repository has moved to a new location:
+https://github.com/Leaflet/Leaflet
+
 Leaflet is a modern open-source JavaScript library for **mobile-friendly interactive maps**.
 It is developed by [Vladimir Agafonkin][] with a team of dedicated [contributors][].
 Weighing just about 27 KB of gzipped JS code, it has all the [features][] most developers ever need for online maps.