add guideline: Node.js version usage & support

[boneskull]

• What version(s) of Node.js should be run in a development, CI, or production environment? Evaluate risk of running EOL Node.js versions in these environments.
• For OSS projects, should the minimum supported Node.js version track the above? When can OSS projects drop an old version of Node.js, when should they, and when must they?
• Any special considerations for odd-numbered releases? These versions do not enter LTS and have a lifespan of about nine months.
• Given Node.js has a documented release cadence--a version going EOL should not be a surprise--how should projects use this info to stay within the guidelines over time?

[mcmire]

We've also had questions around:

• what the engines.node field in package.json represents and how that relates to the current, past, and future versions of Node (considering that Yarn v2+ ignores it by default when installing dependencies)
• what .nvmrc represents and what its relationship to engines.node is
• how the versions of Node we test (i.e. the matrix we use in CI) relate to both of these

Perhaps we ought to use this space to clarify these as well?