Add readme.

Author: twadleigh

README.md

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+# OpenEXR.jl
+Saving and loading of OpenEXR files.
+
+Basic usage:
+```jl
+using OpenEXR
+
+# read an EXR file into an `Array{RGBA{Float16},2}`.
+myimage = OpenEXR.load(File{DataFormat{:EXR}}("myimage.exr"))
+
+# save an image to an EXR file
+OpenEXR.save(File{DataFormat{:EXR}}("myimage2.exr"), myimage)
+```
+
+TODO: get this registered with [FileIO](https://github.com/JuliaIO/FileIO.jl).