readme generates different architecture and node version depending on machine running the command

[chadian]

In the usage section of the generated readme it will generate an example output with a string specifying the system architecture and node version, ie: darwin-x64 node-v10.16.0. This is based on the machine that generates the readme.

The main issue is that this causes PR noise and for those unfamiliar a brief explanation. What would be useful is if this could maybe do one of the following:

1. configured either in the package.json under the oclif key
2. Based on the minimum version specified by the engines key in the package.json

The fallback could still be to use what comes from the machine running the command.