Added comments

Added comments to the huffman spec

src/ImageSharp/Formats/Jpeg/Components/Encoder/HuffmanScanEncoder.cs

@@ -306,6 +306,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
                 {
                     byte b = (byte)(bits >> 24);
                     this.emitBuffer[this.emitLen++] = b;
+
+                    // Adding stuff byte
+                    // This is because by JPEG standard scan data can contain JPEG markers (indicated by the 0xFF byte, followed by a non-zero byte)
+                    // Considering this every 0xFF byte must be followed by 0x00 padding byte to signal that this is not a marker
                     if (b == byte.MaxValue)
                     {
                         this.emitBuffer[this.emitLen++] = byte.MinValue;

src/ImageSharp/Formats/Jpeg/Components/Encoder/HuffmanSpec.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
@@ -27,6 +27,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
                             {
                                0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
                             }),
+
+                // Luminance AC.
                 new HuffmanSpec(
                     new byte[]
                         {
@@ -60,6 +62,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
                             0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
                             0xf9, 0xfa
                         }),
+
+                // Chrominance DC.
                 new HuffmanSpec(
                     new byte[]
                         {
@@ -132,4 +136,4 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
             this.Values = values;
         }
     }
-}
+}