Fix missing "using PixelFormats" line in Readme example (#921)

* Fix missing PixelFormats line in first API example

"<Rgba32>" does not appear to be defined without the "using SixLabors.ImageSharp.PixelFormats;" line and causes a "The type or namespace name 'Rgba32' could not be found (are you missing a using directive or an assembly reference?) (CS0246)" error to occur.

* Remove stray newline

README.md

@@ -72,6 +72,7 @@ On platforms supporting netstandard 1.3+
 ```csharp
 using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.PixelFormats;
 
 // Image.Load(string path) is a shortcut for our default type. 
 // Other pixel formats use Image.Load<TPixel>(string path))