[release-branch.go1.11] doc/go1.11: add link to new WebAssembly wiki page

The wiki page has recently been created, and at this time it's
just a stub. It's expected that support for WebAssembly will be
evolving over time, and the wiki page can be kept updated with
helpful information, how to get started, tips and tricks, etc.

Use present tense because it's expected that there will be more
general information added by the time Go 1.11 release happens.

Also add link to https://webassembly.org/ in first paragraph.

Change-Id: I139c2dcec8f0d7fd89401df38a3e12960946693f
Reviewed-on: https://go-review.googlesource.com/131078


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 6e76aeba)
Reviewed-on: https://go-review.googlesource.com/131096