Add an nvmrc file

[bmuenzenmeyer]

It'd be smart to tell users what Node version we officially support

[tburny]

I would suggest using the latest currently available LTS version here, including the minor version.

[bmuenzenmeyer]

What are you suggesting? updating the engines property in package json to match the nvmrc?

Current LTS is....

https://nodejs.org/download/release/latest-v6.x/
ab99d50#diff-ae9eba3500703eaba1d5b8e13832d955

at least until https://github.com/nodejs/Release switches it to 8 sometime in October....