doc/go1.3.html: new package debug/plan9obj

LGTM=r
R=r
CC=golang-codereviews, rsc
https://golang.org/cl/86180046

doc/go1.3.html

@@ -234,7 +234,8 @@ which can be useful information when debugging deadlocks or performance issues.
 <h3 id="new_packages">New packages</h3>
 
 <p>
-No new packages appear in the core libraries in Go 1.3.
+A new package <a href="/pkg/debug/plan9obj/"><code>debug/plan9obj</code></a> was added to the standard library.
+It implements access to Plan 9 <a href="http://plan9.bell-labs.com/magic/man2html/6/a.out">a.out</a> object files.
 </p>
 
 <h3 id="major_library_changes">Major changes to the library</h3>