Update README to include eslint environment

eslint does not use the plugin if mocha is not mentioned in the environment. Please update the readme to include that in config:

{
  "env": {
    "mocha": true
  },
  "plugins": [
    "mocha"
  ],
  "rules": {
    "mocha/no-exclusive-tests": "error"
  }
}

Similar issue occured in the eslint-plugin-jasmine and environment declaration fixed it.

[mrchess]

+1. I’m new to eslint and it took me forever how to locate that “env” was the issue on why “describe” was not getting read correctly.

[lo1tuma]

Setting the mocha env is not required in order to use this plugin. Please have a look at the eslint documentation about environments. Also the mentioned issue in eslint-plugin-jasmine was not about that the plugin doesn’t work, it was about problems reported by the eslint core rule no-undef.