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/2261


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>