[bootprint-openapi] Add npx command to readme?

[LouisStAmour]

You can skip the global install by doing:

npx -p bootprint-openapi -p bootprint bootprint openapi http://petstore.swagger.io/v2/swagger.json output-folder

That said, npx does not appear to cache any packages if they're specified explicitly at the command line, as above, so the above command does use extra data transfer if you need to run it more than once.