doc: add cmd/go pkgdir changes to go1.8.txt

Change-Id: I451ca386781b50ab47b313e07a610867fa14aeaf
Reviewed-on: https://go-review.googlesource.com/27990Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

doc/go1.8.txt

@@ -4,6 +4,7 @@ Tools:
 compile: SSA for 386, arm, arm64, ppc64le, ... (many CLs)
 yacc: "go tool yacc" is removed. now at golang.org/x/tools/cmd/goyacc (CL 27324, CL 27325)
 go: -buildmode=c-archive now builds PIC on ELF (CL 24180)
+go: mobile pkg dir change, recommend using go list in scripts (CL 24930, CL 27929)
 
 Ports: