doc/go1.15: add release note for encoding/asn1

Updates #37419

Change-Id: I05368efbedd8c7e0b50cd691559491699c3a0945
Reviewed-on: https://go-review.googlesource.com/c/go/+/240201
Reviewed-by: Filippo Valsorda <filippo@golang.org>
diff --git a/doc/go1.15.html b/doc/go1.15.html
index ea0c54b..a0de2b0 100644
--- a/doc/go1.15.html
+++ b/doc/go1.15.html
@@ -595,6 +595,15 @@
   </dd>
 </dl><!-- debug/pe -->
 
+<dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1/">encoding/asn1</a></dt>
+  <dd>
+    <p><!-- CL 226984 -->
+      <a href="/pkg/encoding/asn1/#Marshal">Marshal</a> now sorts the components
+      of SET OF according to X.690 DER.
+    </p>
+  </dd>
+</dl><!-- encoding/asn1 -->
+
 <dl id="encoding/json"><dt><a href="/pkg/encoding/json/">encoding/json</a></dt>
   <dd>
     <p><!-- CL 191783 -->