Update README for exporting GOPATH when building

I failed to build gotags with below message (on ArchLinux):

  $ go install onestepback.org/gotags
  can't load package: package onestepback.org/gotags: cannot find package "onestepback.org/gotags" in any of:
      /usr/lib/go/src/pkg/onestepback.org/gotags (from $GOROOT)
      ($GOPATH not set)

And then found something in Rakefile.  I think it should be mentioned
in README too.