Document where new linting rules go (#23035)

Co-authored-by: Michael Weimann <michaelw@matrix.org>

.eslintrc.js

@@ -23,6 +23,8 @@ module.exports = {
             "plugin:matrix-org/typescript",
             "plugin:matrix-org/react",
         ],
+        // NOTE: These rules are frozen and new rules should not be added here.
+        // New changes belong in https://github.com/matrix-org/eslint-plugin-matrix-org/
         rules: {
             // Things we do that break the ideal style
             "prefer-promise-reject-errors": "off",