doc: add http server trailers support to go1.5.txt

Change-Id: Ic15e40bb1ae0bf3a4e8c43a44daf7cc35a18da87
Reviewed-on: https://go-review.googlesource.com/2192Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.5.txt

 crypto/cipher: clarify what will happen if len(src) != len(dst) for the Stream interface. (https://golang.org/cl/1754)
 crypto/tls: change default minimum version to TLS 1.0. (https://golang.org/cl/1791)
 encoding/base64: add unpadded encodings (https://golang.org/cl/1511)
+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)