godoc: serve robots.txt raw

R=gri, niemeyer, rsc, r2, r
CC=golang-dev
https://golang.org/cl/4188063
diff --git a/robots.txt b/robots.txt
new file mode 100644
index 0000000..1f53798
--- /dev/null
+++ b/robots.txt
@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /
diff --git a/src/cmd/godoc/godoc.go b/src/cmd/godoc/godoc.go
index c91dc33..c0943cd 100644
--- a/src/cmd/godoc/godoc.go
+++ b/src/cmd/godoc/godoc.go
@@ -91,6 +91,7 @@
 	mux.Handle(pkgHandler.pattern, &pkgHandler)
 	mux.HandleFunc("/doc/codewalk/", codewalk)
 	mux.HandleFunc("/search", search)
+	mux.Handle("/robots.txt", fileServer)
 	mux.HandleFunc("/", serveFile)
 }