Main readme has an incorrect command

[tjayrush]

In your main readme, it says I should run a command:

gx-go --rewrite

when I did that I get this error:

@./test:jrush>$ gx-go --rewrite
Incorrect Usage. flag provided but not defined: -rewrite

NAME:
   gx-go - gx extensions for golang

USAGE:
   gx-go [global options] command [command options] [arguments...]

VERSION:
   1.9.0

AUTHOR:
   whyrusleeping

COMMANDS:
     dep-map      prints out a json dep map for usage by 'import --map'
     hook         go specific hooks to be called by the gx tool
     import       import a go package and all its depencies into gx
     path         prints the import path of the current package within GOPATH
     rewrite, rw  temporary hack to evade causality
     uw
     update       update a packages imports to a new path
     dvcs-deps    display all dvcs deps
     link         Symlink packages to their dvcsimport repos, for local development.
     lock-gen     Generate a lock file
     devcopy      Create a development copy of the given package
     get          gx-ified `go get`
     help, h      Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --verbose      turn on verbose output
   --help, -h     show help
   --version, -v  print the version
ERROR: Error: flag provided but not defined: -rewrite

[Stebalien]

I've removed all mentions of gx entirely. If/when gx matures to the point where we can reasonably recommend it, we can add it back.