doc: update go1.6.txt

Mention shallow clones.

Fixes #13204

Change-Id: I0ed9d4e829d388425beba0d64e6889d16d4bb173
Reviewed-on: https://go-review.googlesource.com/16822Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.6.txt

@@ -3,6 +3,7 @@ Tools:
 cmd/go: vendoring enabled by default (https://golang.org/cl/13967/)
 cmd/go: flags for tests must precede package name if present; also makes it easier to pass flags to test binaries (https://golang.org/cl/14826)
 cmd/go: add -msan option (https://golang.org/cl/16169)
+cmd/go: use shallow clones for new git checkouts (https://golang.org/cl/16360)
 cmd/compile: add -msan option (https://golang.org/cl/16160)
 cmd/link: add -msan option (https://golang.org/cl/16161)