doc/go1.3.html: Plan 9 support

LGTM=aram, r
R=r, aram, bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/89130045

doc/go1.3.html

@@ -14,7 +14,7 @@ precise garbage collection,
 a major refactoring of the compiler tool chain that results in
 faster builds, especially for large projects,
 significant performance improvements across the board,
-and support for Solaris and Google's Native Client architecture (NaCl).
+and support for Solaris, Plan 9 and Google's Native Client architecture (NaCl).
 It also has an important refinement to the memory model regarding synchronization.
 As always, Go 1.3 keeps the <a href="/doc/go1compat.html">promise
 of compatibility</a>,
@@ -42,6 +42,11 @@ how to set up the Go version is described <a href="http://golang.org/wiki/Native
 Go 1.3 now includes experimental support for Solaris on the <code>amd64</code> (64-bit x86) architecture.
 </p>
 
+<h3 id="plan9">Support for Plan 9</h3>
+
+<p>
+Go 1.3 now includes experimental support for Plan 9 on the <code>386</code> (32-bit x86) architecture.
+</p>
 
 <h3 id="win2000">Removal of support for Windows 2000</h3>