doc/go1.5.txt: LookupEnv

Change-Id: Iff27fa0ca50fe9e41d811d30df41fc2d3057aa1d
Reviewed-on: https://go-review.googlesource.com/9792Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.txt

@@ -56,6 +56,7 @@ net/http: support for setting trailers from a server Handler (https://golang.org
 net/http: ignore the Unix epoch time in ServeContent (https://golang.org/cl/7915)
 net/http/cgi: fix REMOTE_ADDR, REMOTE_HOST, add REMOTE_PORT (https://golang.org/cl/4933)
 net/smtp: add TLSConnectionState accessor (https://golang.org/cl/2151)
+os: add LookupEnv (https://golang.org/cl/9791)
 os/signal: add Ignore and Reset (https://golang.org/cl/3580)
 reflect: add ArrayOf (https://golang.org/cl/4111)
 reflect: add FuncOf (https://golang.org/cl/1996)