geojson crs is staying after all

I propose removing the caveat about GeoJSON CRS, as it looks like the GeoJSON WG has determined that removing the CRS definition from the standard would cause more problems than it would solve.  So, `L.Proj.GeoJSON` will continue be an implementation of a standard (though not recommended) GeoJSON feature - the ability to use coordinate systems other than CRS84.

See https://github.com/geojson/draft-geojson/issues/14 and https://gist.github.com/sgillies/8304592.

cc @mourner