doc: document math/big performance improvements

Change-Id: I2b40cd544dda550ac6ac6da19ba3867ec30b2774
Reviewed-on: https://go-review.googlesource.com/2563Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go1.5.txt

@@ -11,3 +11,5 @@ net/smtp: add TLSConnectionState accessor (https://golang.org/cl/2151)
 Performance:
 
 strconv: optimize decimal to string conversion (https://golang.org/cl/2105)
+math/big: faster assembly kernels for amd64 and 386 (https://golang.org/cl/2503, https://golang.org/cl/2560)
+math/big: faster "pure Go" kernels for platforms w/o assembly kernels (https://golang.org/cl/2480)