Add deprecation notice to README

[bmish]

Using the ember-cli linting plugins has been deprecated for years based on the RFC linked to below. ember-cli-eslint and ember-cli-template-lint have already been marked deprecated and have deprecation notices in their READMEs as linked to below.

In this PR, I'm proposing/implementing a deprecation notice suggesting to use stylelint directly instead. If this is accepted, the maintainer should also run the following command to mark the package as deprecated in the NPM registry:

npm deprecate ember-cli-stylelint "Use stylelint directly instead: https://github.com/emberjs/rfcs/blob/master/text/0121-remove-ember-cli-eslint.md"

By using stylelint directly, we mean that Ember apps should tell their CI to run their lint script the same way it runs their test script. The lint scripts in package.json usually look roughly like this in Ember apps (simplified version from ember-cli blueprint):

"lint": "npm-run-all --aggregate-output --continue-on-error --parallel lint:*",
"lint:css": "stylelint .",
"lint:hbs": "ember-template-lint .",
"lint:js": "eslint .",

More info:

• [RFC] Replace ember-cli-eslint with the standard eslint ember-cli/rfcs#121
• https://github.com/emberjs/rfcs/blob/master/text/0121-remove-ember-cli-eslint.md
• Use eslint and ember-template-lint directly (no longer lint during builds/rebuilds by default) ember-cli/ember-cli#9009
• Add deprecation notice to README ember-cli/ember-cli-eslint#597
• https://github.com/ember-cli/ember-cli-eslint#-deprecated-
• https://github.com/ember-template-lint/ember-cli-template-lint#-deprecated-
• https://www.npmjs.com/package/ember-cli-eslint
• https://www.npmjs.com/package/ember-cli-template-lint

[bmish]

@billybonks mind if we merge this?