doc: mention installation in README

Fixes #9976

Change-Id: I57f4d3526f7cd5aaf8d3d7e6b6b09843398d5d32
Reviewed-on: https://go-review.googlesource.com/7617
Reviewed-by: Minux Ma <minux@golang.org>

README

@@ -1,5 +1,9 @@
 This is Go Tour, an introduction to the Go programming language.
 
+To install the tour locally, first install Go, then run:
+
+	$ go get golang.org/x/tour/gotour
+
 Unless otherwise noted, the go-tour source files are distributed
 under the BSD-style license found in the LICENSE file.