doc: mention net.OpError in go1.5.txt

Change-Id: I6cebaf42f2596c7f8fef3a67afb1e5ccb428d09c
Reviewed-on: https://go-review.googlesource.com/9521Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.5.txt

@@ -47,6 +47,7 @@ log: add LUTC flag (https://golang.org/cl/8761)
 math/big: add arbitrary precision Floats (many cl's)
 mime: add ExtensionByType (https://golang.org/cl/7444)
 mime/quotedprintable: new package (https://golang.org/cl/5940 + others)
+net: add Source field to OpError (https://go-review.googlesource.com/9231)
 net/http: support for setting trailers from a server Handler (https://golang.org/cl/2157)
 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)