add vendor & update README.md

vendor/github.com/mattn/go-isatty/isatty_appengine.go

@@ -0,0 +1,15 @@
+// +build appengine
+
+package isatty
+
+// IsTerminal returns true if the file descriptor is terminal which
+// is always false on on appengine classic which is a sandboxed PaaS.
+func IsTerminal(fd uintptr) bool {
+	return false
+}
+
+// IsCygwinTerminal() return true if the file descriptor is a cygwin or msys2
+// terminal. This is also always false on this environment.
+func IsCygwinTerminal(fd uintptr) bool {
+	return false
+}