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

Change-Id: I103a21ae0be8feb9447fa277483b7c2a8b5807e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/188540
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>
Reviewed-by: Richard Musiol <eb533a4b7e679e5143b37a0022300174aaa46b21@gmail.com>

nawawi/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,golang/go,golang/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,akutz/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go