doc/install: add openbsd

Add openbsd to the $GOOS list.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/4964053

doc/install.html

@@ -61,7 +61,7 @@ support for segmented stacks, and a strong goroutine implementation.
 </p>
 
 <p>
-The compilers can target the FreeBSD, Linux,
+The compilers can target the FreeBSD, Linux, OpenBSD
 and OS X (a.k.a. Darwin) operating systems.
 (A port to Microsoft Windows is in progress but incomplete.  See the 
 <a href="http://code.google.com/p/go/wiki/WindowsPort">Windows Port</a>
@@ -373,9 +373,9 @@ to override the defaults.
 	<code>$GOHOSTARCH</code> respectively (described below).
 
 	<p>
-	Choices for <code>$GOOS</code> are <code>linux</code>,
-	<code>freebsd</code>,
+	Choices for <code>$GOOS</code> are
 	<code>darwin</code> (Mac OS X 10.5 or 10.6),
+	<code>freebsd</code>, <code>linux</code>, <code>openbsd</code>,
 	and <code>windows</code> (Windows, an incomplete port).
 	Choices for <code>$GOARCH</code> are <code>amd64</code> (64-bit x86, the most mature port),
 	<code>386</code> (32-bit x86), and
@@ -407,6 +407,12 @@ to override the defaults.
 	<td></td><td><code>linux</code></td> <td><code>arm</code></td> <td><i>incomplete</i></td>
 	</tr>
 	<tr>
+	<td></td><td><code>openbsd</code></td> <td><code>386</code></td>
+	</tr>
+	<tr>
+	<td></td><td><code>openbsd</code></td> <td><code>amd64</code></td>
+	</tr>
+	<tr>
 	<td></td><td><code>windows</code></td> <td><code>386</code></td> <td><i>incomplete</i></td>
 	</tr>
 	</table>