doc: mention vendoring in go1.6.txt (forgot git add before)

Change-Id: I34eff138c61e5ad456a4918c402ca0e8333601a0
Reviewed-on: https://go-review.googlesource.com/13978Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.6.txt

+Tools:
+
+cmd/go: vendoring enabled by default (https://golang.org/cl/13967/)
+
 Ports:
 
 NaCl is no longer restricted to pepper_41 (https://golang.org/cl/13958/)