doc/go1.5.txt: mention goroutine scheduling change

Change-Id: I43a40f0c10472b3126c06b0d4268b32b54665f12
Reviewed-on: https://go-review.googlesource.com/9349Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.txt

@@ -13,6 +13,9 @@ darwin/arm, a.k.a iOS. (https://golang.org/cl/2118, 2119, 3273, 2121, 2122, ...,
 darwin/arm64
 linux/arm64 (cgo is supported, but only with external linking)
 
+Runtime:
+goroutine scheduling order changed; never guaranteed by language, but can break tests that implicitly assume a specific execution order
+
 Removed Ports:
 dragonfly/386 (https://golang.org/cl/7543)