doc/go1.5.txt: mention zip.WriterAt

Change-Id: I9f5ca101e0d2fe71cb1ca810cfeeb82c12f5f8e7
Reviewed-on: https://go-review.googlesource.com/7491Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.5.txt

@@ -7,6 +7,7 @@ Darwin/ARM, a.k.a iOS. (https://golang.org/cl/2118, 2119, 3273, 2121, 2122, ...,
 
 API additions and behavior changes:
 
+archive/zip: add WriterAt function (https://golang.org/cl/7445)
 bufio: add Reader.Discard (https://golang.org/cl/2260)
 crypto/cipher: clarify what will happen if len(src) != len(dst) for the Stream interface. (https://golang.org/cl/1754)
 crypto/elliptic: add Name field to CurveParams struct (https://golang.org/cl/2133)