doc/go1.5.txt: mention GOARCH reservation change

Change-Id: Ie0ba7a5cb860e6fff68ec3042764c7e026b1234c
Reviewed-on: https://go-review.googlesource.com/9781Reviewed-by: Minux Ma <minux@golang.org>

doc/go1.5.txt

@@ -43,6 +43,7 @@ flag: new nicer format for PrintDefaults (https://golang.org/cl/7330)
 fmt: empty slices now print nothing with %x (bug fix) (https://golang.org/cl/8864)
 fmt: reflect.Value now prints what it holds (https://golang.org/cl/8731)
 go/ast: add Implicit field to ast.EmptyStmt; changed meaning of ast.EmptyStmt.Semicolon position (https://golang.org/cl/5720)
+go/build: reserved GOARCHes for common architectures (https://golang.org/cl/9644)
 io: add CopyBuffer, Copy with user-provided buffer (https://golang.org/cl/8730)
 log: add SetOutput functions (https://golang.org/cl/2686, https://golang.org/cl/3023)
 log: add LUTC flag (https://golang.org/cl/8761)