soft deprecate configuration via mocha.opts

lib/cli/options.js

@@ -200,7 +200,7 @@ module.exports = function getOptions() {
 /**
  * Given filepath in `args.opts`, attempt to load and parse a `mocha.opts` file.
  * @param {Object} [args] - Arguments object
- * @param {string|boolean} [args.opts] - Filepath to mocha.opts; defaults to whatever's in `mocharc.opts`, or `false` to skip
+ * @param {string|boolean} [args.opts] - Filepath to mocha.opts; defaults to whatever's in `mocharc.json`, or `false` to skip
  * @returns {external:yargsParser.Arguments|void} If read, object containing parsed arguments
  * @memberof module:lib/cli/options
  * @public
@@ -230,6 +230,9 @@ const loadMochaOpts = (args = {}) => {
   // if there's an exception to catch here, I'm not sure what it is.
   // by attaching the `no-opts` arg, we avoid re-parsing of `mocha.opts`.
   if (mochaOpts) {
+    deprecate(
+      'Configuration via mocha.opts is DEPRECATED and will be removed from a future version of Mocha. Use RC files or package.json instead.'
+    );
     result = parse(parseMochaOpts(mochaOpts));
     debug(`${filepath} parsed succesfully`);
   }