Add readonly annotations

src/ImageSharp/MetaData/Profiles/Exif/ExifTagDescriptionAttribute.cs

@@ -12,8 +12,8 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Exif
     [AttributeUsage(AttributeTargets.Field, AllowMultiple = true)]
     internal sealed class ExifTagDescriptionAttribute : Attribute
     {
-        private object value;
-        private string description;
+        private readonly object value;
+        private readonly string description;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ExifTagDescriptionAttribute"/> class.

src/ImageSharp/MetaData/Profiles/Exif/ExifWriter.cs

@@ -20,9 +20,9 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Exif
         private ExifParts allowedParts;
         private IList<ExifValue> values;
         private List<int> dataOffsets;
-        private List<int> ifdIndexes;
-        private List<int> exifIndexes;
-        private List<int> gpsIndexes;
+        private readonly List<int> ifdIndexes;
+        private readonly List<int> exifIndexes;
+        private readonly List<int> gpsIndexes;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ExifWriter"/> class.

src/ImageSharp/MetaData/Profiles/ICC/IccProfile.cs

@@ -18,7 +18,7 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Icc
         /// <summary>
         /// The byte array to read the ICC profile from
         /// </summary>
-        private byte[] data;
+        private readonly byte[] data;
 
         /// <summary>
         /// The backing file for the <see cref="Entries"/> property