doc: add a CL to go1.8.txt mentioned by Alberto Donizetti

Change-Id: I43617e6dfd5b8227a8ef907dc22c00188de87b94
Reviewed-on: https://go-review.googlesource.com/32915
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz

doc/go1.8.txt

@@ -396,6 +396,7 @@ image/color: improve speed of RGBA methods (CL 31773)
 image/draw: optimize drawFillOver as drawFillSrc for opaque fills (CL 28790)
 math/big: avoid allocation in float.{Add, Sub} when there's no aliasing (CL 23568)
 math/big: make division faster (CL 30613)
+math/big: slightly faster float->decimal conversion (CL 31250)
 math/big: use array instead of slice for deBruijn lookups (CL 26663)
 math/big: uses SIMD for some math big functions on s390x (CL 32211)
 math: speed up Gamma(+Inf) (CL 31370)