Add as part of FAQ: 'how do I require a library with fs access'

[brian-mann]

We commonly see users try to do this: cypress-io/cypress#1963

They want to require an npm lib that has fs access in it. They can't do that, but it's easy to simply use cy.task or something else in order to access the background node process.

Need to make it clear / findable / searchable to do this.

[gelicia]

I found this issue (and related) trying to figure out why cypress was finding my plugins file as a spec, and then failing on the fs-extra command using the code here to give myself multiple config files. I later found out that my main problem was my pluginsFile was in a folder nested inside my integration folder. This also keeps ignoreTestFiles from working. Anyway, this is only tangentially related but if you're updating docs there might be a spot for "be sure the plugins folder is a sibling of your integrationFolder."