doc: mention that go install removes binaries built by go build

Fixes #12288.

For inclusion in the 1.5.1 release.

Change-Id: I9354b7eaa76000498465c4a5cbab7246de9ecb7c
Reviewed-on: https://go-review.googlesource.com/14382Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.5.html

@@ -504,6 +504,13 @@ SWIG support has been updated such that
 now require SWIG 3.0.6 or later.
 </li>
 
+<li>
+The <code>install</code> subcommand now removes the
+binary created by the <code>build</code> subcommand
+in the source directory, if present,
+to avoid problems having two binaries present in the tree.
+</li>
+
 <li>
 The <code>std</code> (standard library) wildcard package name
 now excludes commands.