Add reference to design document (#8428)

Author: RufusJWB

xml/ns-System.Formats.Asn1.xml

@@ -1,6 +1,12 @@
 <Namespace Name="System.Formats.Asn1">
   <Docs>
     <summary>Contains types used in reading and writing Abstract Syntax Notation One (ASN.1) data structures.</summary>
-    <remarks>To be added.</remarks>
+    <remarks>
+      <format type="text/markdown"><![CDATA[
+## Remarks
+
+For more information, see the [ASN.1 BER/CER/DER reader and writer design document](https://github.com/dotnet/designs/blob/main/accepted/2020/asnreader/asnreader.md).
+      ]]></format>
+    </remarks>
   </Docs>
 </Namespace>