Add NOTICE file with info about vendored dependencies

[watson]

The Node.js agent have currently vendored two files from two different GitHub repos:

https://github.com/elastic/apm-agent-nodejs/blob/master/lib/instrumentation/patch-async.js
https://github.com/elastic/apm-agent-nodejs/blob/master/lib/instrumentation/shimmer.js

While both files are licenses the same as this repo, and both contain a comment indicating this, it would be best if we added a NOTICE file in the root of the repo specifying this.