readme improvements

ios/media/coreimage/adjust_contrast_and_brightness_of_an_image/color_controls/readme.md

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+This is the sample code for the iOS recipe for adjusting the brightness and contrast of images.
+
+[See the recipe at developer.xamarin.com](http://developer.xamarin.com/recipes/ios/media/coreimage/adjust_contrast_and_brightness_of_an_image/)