doc/go1.8.txt: mention os.Executable addition

Change-Id: Id3d571666b9275e3fa5cb20762afbd391dbcdeba
Reviewed-on: https://go-review.googlesource.com/32883Reviewed-by: Minux Ma <minux@golang.org>

doc/go1.8.txt

@@ -259,6 +259,7 @@ net: there are no invalid domain names anymore (CL 31468)
 net: use libresolv rules for ndots range and validation (CL 24901)
 os, syscall: fix incorrect offset calculation in Readlink on windows (CL 31118)
 os: add ErrClosed, return for use of closed File (CL 30614)
+os: add Executable() (CL 16551)
 os: consider only files from #M as regular on Plan 9 (CL 32152)
 os: don't let File.Readdir return an empty slice and nil error (CL 28056)
 os: make IsExist report true on ERROR_DIR_NOT_EMPTY on Windows (CL 29753)