doc: mention 'go get golang.org/dl/...' in install

I needed Go 1.10 to debug and fix a test failure on that Go version in
x/tools, but I forgot what the magic 'go get' command for this was.

Googling "download specific golang version" and similar keywords showed
no results, presumably because the golang.org/dl subrepo isn't
prominently recommended nor documented anywhere.

The most appropriate documentation page to add this to is doc/install,
since it goes into some detail and is well indexed. We only need a short
section to introduce the trick.

The example does mention a specific version, Go 1.10.7, but I couldn't
imagine a way to make it version-agnostic while still being clear on
what the commands effectively do.

Change-Id: I13158564d76d95caec412cdb35a50a4356df5863
Reviewed-on: https://go-review.googlesource.com/c/157457


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>