add installation instructions

Author: chrisbreiding

README.md

@@ -4,10 +4,17 @@ An ESLint plugin for projects that use Cypress.
 
 Specifies the environment and globals (`cy`, `Cypress`, etc) for use when writing tests using [Cypress](https://cypress.io).
 
+## Installation
+
+```sh
+npm install eslint-plugin-cypress --save-dev
+```
+
 ## Usage
 
-```js
-// .eslintrc
+Add `cypress` as a plugin in your `.eslintrc`:
+
+```json
 {
   "plugins": [
     "cypress"