doc: Add reminder to go1.5.txt for os/signal changes

Document addition of Ignore and Reset: https://golang.org/cl/3580

Change-Id: I33aac149cd1921b87f887028c0365ba0ab9adb02
Reviewed-on: https://go-review.googlesource.com/4980Reviewed-by: Minux Ma <minux@golang.org>

doc/go1.5.txt

@@ -15,6 +15,7 @@ encoding/base64: add unpadded encodings (https://golang.org/cl/1511)
 log: add SetOutput functions (https://golang.org/cl/2686, https://golang.org/cl/3023)
 net/http: support for setting trailers from a server Handler (https://golang.org/cl/2157)
 net/smtp: add TLSConnectionState accessor (https://golang.org/cl/2151)
+os/signal: add Ignore and Reset (https://golang.org/cl/3580)
 runtime, syscall: use SYSCALL instruction on FreeBSD (Go 1.5 now requires FreeBSD 8-STABLE+) (https://golang.org/cl/3020)
 strings: add Compare(x, y string) int, for symmetry with bytes.Compare (https://golang.org/cl/2828)
 testing/quick: support generation of arrays (https://golang.org/cl/3865)