doc/go1.5.txt: add Reader.Size to bytes and strings

Change-Id: Idd42e0f5c6ed55be2e153ac83022439e5272c1a7
Reviewed-on: https://go-review.googlesource.com/8444Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

doc/go1.5.txt

@@ -20,6 +20,7 @@ API additions and behavior changes:
 archive/zip: add (*Writer).SetOffset method (https://golang.org/cl/7445)
 bufio: add Reader.Discard (https://golang.org/cl/2260)
 bytes: add Buffer.Cap (https://golang.org/cl/8342)
+bytes, strings: add Reader.Size (https://golang.org/cl/3199)
 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)
 crypto/tls: change default minimum version to TLS 1.0. (https://golang.org/cl/1791)