add info about globals

[jpratt2]

Hello, I get a 404 when I try to edit https://nightwatchjs.org/guide/writing-tests/using-test-globals.html
May I suggest an update:
to add a global variable to the globals.js file, you can point out that it is not necessary to use the "module.exports" section. Just enter your variable above that section, like regular code:

myGlobal = 3000;

[garg3133]

@jpratt2 Not sure what you mean here. Can you submit a PR for this? The file you're looking for is here: https://github.com/nightwatchjs/nightwatch-docs/blob/versions/3.0/docs/guide/writing-tests/using-test-globals.md