Update README.md

Added Contributing info.
Added a 'convert' function to RasterSource
Added fromString to Extent
Doc string was wrong.

The problem with doc strings is that they can sometimes be misleading!
Merge pull request #691 from lossyrob/0.9

Merging in master commits that should go in 0.9
Merge pull request #692 from lossyrob/RasterSource/convert

Added a 'convert' function to RasterSource
Created CanBuildFrom from ValueSource[_] -> RasterSource

This is for mapping a ValueSource to a raster, for instance if you
rasterize a polygon, you should get a RasterSource back.

Changed the .info on RasterSourceLike to check if the definition is
for a catalogued raster; if so, pull the info from the catalog, else
build an info off of the raster definition.
Merge branch 'ValueSource/RasterSource' into 0.9
Merge branch 'RasterSource/convert' into 0.9
Moved version to RC2
Changed renderPng with no params to assume it's a RBGA raster.
Merge branch 'RenderOpMethods/render-color-raster' into 0.9