Add contributing

CONTRIBUTING.md

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+Before you post an issue, please review this document! Many of your questions,
+like "why isn't leaflet-image working" or "will it work with a plugin",
+are answered below.
+
+## Requirements
+
+* Tile layer providers (OSM, MapBox, etc) must support [CORS](http://en.wikipedia.org/wiki/Cross-origin_resource_sharing)
+* Any markers on the map must also support CORS. The default Leaflet-CDN markers
+  don't, so they aren't supported.
+* Your browser must support [CORS](http://caniuse.com/#feat=cors) and [Canvas](http://caniuse.com/#feat=canvas),
+  so `IE >= 10` with no exceptions.
+* This library **does not rasterize HTML** because **browsers cannot rasterize HTML**. Therefore,
+  L.divIcon and other HTML-based features of a map, like zoom controls or legends, are not
+  included in the output, because they are HTML.
+
+__For Leaflet < 1.0.0__: You must set `L_PREFER_CANVAS = true;` so that vector
+  layers are drawn in Canvas
+
+__For Leaflet >= 1.0.0__: You must set `renderer: L.canvas()` for any layer that
+  you want included in the generated image. You can also set this by setting [`preferCanvas: true`](http://leafletjs.com/reference-1.0.0.html#map-prefercanvas) in your map's options.
+
+## Plugins that will _not_ work with leaflet-image
+
+* Leaflet.label: will not work because it uses HTML to display labels.