Added doc description for the NSData category

@@ -6,5 +6,14 @@
 #import <Foundation/Foundation.h>
 
 @interface NSData (ImageContentType)
+
+/**
+ *  Compute the content type for an image data
+ *
+ *  @param data the input data
+ *
+ *  @return the content type as string (i.e. image/jpeg, image/gif)
+ */
 + (NSString *)contentTypeForImageData:(NSData *)data;
+
 @end