doc: note that plugins are not fully baked

Change-Id: I6341b8cce0b4a9922928f73f8b459cbb9ec25e79
Reviewed-on: https://go-review.googlesource.com/35571Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

doc/go1.8.html

@@ -435,11 +435,11 @@ version of gccgo.
 <h3 id="plugin">Plugins</h3>
 
 <p>
-  Go now supports a “<code>plugin</code>” build mode for generating
-  plugins written in Go, and a
+  Go now provides early support for plugins with a “<code>plugin</code>”
+  build mode for generating plugins written in Go, and a
   new <a href="/pkg/plugin/"><code>plugin</code></a> package for
-  loading such plugins at run time. Plugin support is only currently
-  available on Linux.
+  loading such plugins at run time. Plugin support is currently only
+  available on Linux. Please report any issues.
 </p>
 
 <h2 id="runtime">Runtime</h2>