Document the eslint update process #12611

See #12563 (comment)

As is, I'm OK with putting eslint-plugin-markdown into tools/eslint/node_modules as long as things break in an understandable way when the plugin is missing after an upgrade. For that matter, the upgrade process probably needs to be documented somewhere and that would be a good place to include the instructions to include the plugin. I might try to do that if I get some time today, but as always, I'd be thrilled if someone beats me to it. (I always forget a step in the upgrade anyway involving a symlink or something to .bin/eslint.js I think?

The symlink step was done in #10771 and #9299

See also #9299 (comment)

@Trott I'm not sure either. How did you create the eslint you committed? What was your sequence? We could probably figure out one that works and document in a README somewhere. Or in the next commit message. You put some effort into your commit to say what you did, but just at a high level.

cc/ @Trott @vsemozhetbyt @sam-github