doc: add Nodeiflux Discord Community to README

[Trott]

I don't want the list of unofficial resources to become a laundry list
of everything that exists, but it seems like a Discord community belongs
there...

@vcarl

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

[nodejs-github-bot]

@Trott build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/3007/pipeline

[Trott]

I'm massively Discord-ignorant and can't even answer "yes" or "no" to simple questions like "Does this Discord have a Code of Conduct?" Or "Is there another Node community on Discord of comparable or greater size/activity?"

If someone else is a sufficiently active user to have a good idea of the answers to those sorts of questions and wants to see this land, feel free to open a PR. (This one is one line and you can totally copy it.)

[vcarl]

To my knowledge there isn't a specifically node.js focused Discord that's larger, though Speak.js has more members.

As for a CoC, we operate under the Reactiflux guidelines but I've been dragging my feet on getting it set up under a nodeiflux.com site (though I own the domain).