Document supported Node.js version

[mcandre]

Please clarify which Node.js versions are supported by editorconfig-tools!

• [ ] package.json -> "engine" key
• [ ] .node-version file

editorconfig-tools appears to work with Node.js 4.5 on my machine

[glensc]

node 14 fails with:

$ editorconfig-tools check rules/*.yml
fs.js:40
} = primordials;