Clarify supported node.js versions in Readme.md

[truedat101]

If N-api bindings are used, this implies Node 9 or 10. Node 8.x would not be supported unless N-api gets back ported, right?

[no2chem]

Yes. This didn't seem that important to me as Node <9? doesn't support bigints anyway, but will clarify that in the readme, thanks!

[truedat101]

And it was an obvious mistake on my part trying to install with node 8. The build and run warning was a clear sign N-api was a dependency, and the node-gyp build warned. The Int64 not yet fully supported in the javascript platform was news to me. So contemplating moving to Node 10.

[khoazany]

I think Node version clarification is needed as this currently can't run on AWS Lambda due to them only have Node 6/8 as possible environment.