doc: add a section for performance improvements in go1.5.txt

Mostly I need to tickle the builders, since I'm working on the
dashboard builders right now.

Change-Id: I833fc22bc942758a58791ed038634cdd812f5411
Reviewed-on: https://go-review.googlesource.com/2261Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.5.txt

+API additions and behavior changes:
+
 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)
+
+
+Performance:
+
+strconv: optimize decimal to string conversion (https://golang.org/cl/2105)