Update README to specify which node version is needed

[aajacobs]

Please indicate that you followed the steps below before filing an issue:

• [ X ] Followed the steps in the Troubleshooting section of the README.
• [ X ] If it is a bug, retrieved any errors that may be thrown in the browser, in console by node, or by MarkLogic.

Using Node 10.4 resulted in errors with npm install (fs-event errors) and gulp-serve local. Moving to LTS fixed the issues (which is 8.11.2 for me as of now).