Add node.js version dependancy

[mandys]

On one of our test servers, I had an old version of nodejs. The code did not work and no errors were reported.

It worked fine when I updated to node 10.10.0.

I don't know the minimum requirements, but would be helpful to others, in case it's added to the documentation.

[catdad]

Sorry about the trouble you had. I have no immediate plans to work on this module at this time. It is not uncommon to assume that a module only supports currently active versions of node (today, that would be node 12 and up). It's still a good idea to include this information, however, so I will leave this open in case someone wants to contribute it.