doc/go1.13: add release notes for GOWASM environment variable

Change-Id: I103a21ae0be8feb9447fa277483b7c2a8b5807e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/188540Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>

doc/go1.13.html

@@ -105,6 +105,11 @@ Do not send CLs removing the interior tags from such phrases.
   Go 1.13 is the last release that will run on Native Client (NaCl).
 </p>
 
+<p><!-- CL 170119, CL 168882 -->
+  For <code>GOARCH=wasm</code>, the new environment variable <code>GOWASM</code> takes a comma-separated list of experimental features that the binary gets compiled with.
+  The valid values are documented <a href="/cmd/go/#hdr-Environment_variables">here</a>.
+</p>
+
 <h3 id="aix">AIX</h3>
 
 <p><!-- CL 164003, CL 169120 -->