[release-branch.go1.7] godoc/dl: add instructions for unstable "go get"

Adds instructions for "go get golang.org/x/build/version/..."
for the unstable version.

Fixes golang/go#18337.

Change-Id: Iea5e8fab7f00a053e3b42ef355277307addb635b
Reviewed-on: https://go-review.googlesource.com/34823
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/34826

godoc/dl/tmpl.go

@@ -213,6 +213,11 @@ $(document).ready(function() {
 		<h2 class="toggleButton" title="Click to hide downloads for this version">{{.Version}} ▾</h2>
 		{{if .Stable}}{{else}}
 			<p>This is an <b>unstable</b> version of Go. Use with caution.</p>
+			<p>If you already have Go installed, you can install this version by running:</p>
+<pre>
+go get golang.org/x/build/version/{{.Version}}
+</pre>
+			<p>Then, use the <code>{{.Version}}</code> command instead of the <code>go</code> command to use {{.Version}}.</p>
 		{{end}}
 		{{template "files" .Files}}
 	</div>