Make frames resonly

Author: JimBobSquarePants

src/ImageSharp/Image.cs

@@ -99,7 +99,7 @@ public void Dispose()
         /// </summary>
         /// <param name="stream">The stream to save the image to.</param>
         /// <param name="encoder">The encoder to save the image with.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown if the stream or encoder is null.</exception>
+        /// <exception cref="ArgumentNullException">Thrown if the stream or encoder is null.</exception>
         public void Save(Stream stream, IImageEncoder encoder)
         {
             Guard.NotNull(stream, nameof(stream));

src/ImageSharp/Image{TPixel}.cs

@@ -23,7 +23,7 @@ namespace SixLabors.ImageSharp
     public sealed class Image<TPixel> : Image
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        private ImageFrameCollection<TPixel> frames;
+        private readonly ImageFrameCollection<TPixel> frames;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Image{TPixel}"/> class