Add System.Runtime.CompilerServices.ReadOnlyAttribute (#10777)

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: OmarTawfik

src/Common/src/CoreLib/System.Private.CoreLib.Shared.projitems

@@ -305,6 +305,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\LoadHint.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\MethodCodeType.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\MethodImplOptions.cs"/>
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ReadOnlyAttribute.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\StringFreezingAttribute.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\SuppressIldasmAttribute.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\TupleElementNamesAttribute.cs"/>

src/Common/src/CoreLib/System/Runtime/CompilerServices/ReadOnlyAttribute.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.ComponentModel;
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Reserved to be used by the compiler for tracking metadata.
+    /// This attribute should not be used by developers in source code.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [AttributeUsage(AttributeTargets.All, Inherited = false)]
+    public sealed class ReadOnlyAttribute : Attribute
+    {
+        public ReadOnlyAttribute()
+        {
+        }
+    }
+}