Add loading indicator.

assets/libs/leaflet-loading/CONTRIBUTING.md

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+Contributing
+============
+
+Your pull requests are very welcome! Please follow these guidelines:
+
+ * Keep code compatible with the same browsers Leaflet aims to be [compatible
+   with](http://leafletjs.com/features.html).
+ * Use four spaces rather than tabs.
+
+
+## Contributors
+
+ * [Eric Brelsford](https://github.com/ebrelsford)
+ * [Nick Iaconis](https://github.com/codefox421)
+ * [Matthias Althaus](https://github.com/althaus)
+ * [Yohan Boniface](https://github.com/yohanboniface)
+ * [kermit-the-frog](https://github.com/kermit-the-frog)
+ * [kcwu](https://github.com/kcwu)
+ * [Robbie Trencheny](https://github.com/robbiet480)