doc/go1.3.html: mention go build -i

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://golang.org/cl/95010049

doc/go1.3.html

@@ -168,6 +168,12 @@ <h3 id="gocmd">Changes to the go command</h3>
 Previously, it would do nothing if no test files were present.
 </p>
 
+<p>
+The <a href="/cmd/go/"><code>go build</code></a> subcommand
+supports a new <code>-i</code> option to install dependencies
+of the specified target, but not the target itself.
+</p>
+
 <p>
 Finally, the go command now supports packages that import Objective-C
 files (suffixed <code>.m</code>) through cgo.