go: documentation for new command

*** This is a design review, not a code review. ***
Feel free to reply to the mail instead of picking out
individual lines to comment on in Rietveld.

This command, go, will replace both gomake/make and goinstall.
Make will stick around only for building our C commands
and perhaps package runtime.

In normal use while developing you'd run commands like

        go compile
        go test
        go clean
        go install

which apply to the package in the current directory.

To operate on code written by others, you add an explicit
package path:

        go get gopath.googlecode.com/hg/oauth
        go test gopath.googlecode.com/hg/oauth

The script.txt file is a script showing the output of
the various help commands that the command has.
(Right now, all the command can do is print help messages.)

R=golang-dev, bradfitz, kevlar, r, edsrzf, gri, adg, rogpeppe, r
CC=golang-dev
https://golang.org/cl/5019045