README npm install command assumes a public module

[rrthomas]

The install command output to README.md is:

npm install -g NAME_OF_MODULE

This doesn't work for me since the module I'm working on is unpublished.

I see there are various issues open about other aspects of using a private repository; perhaps this issues depends on those? Since in fact the project I'm working on is not in any repository, it's simplest just to do:

npm install -g`

in a git checkout. It would be nice if I could tell oclif-dev readme this.