Add docs

docs/rules/prefer-date-now.md

@@ -1,17 +1,37 @@
-# Prefer `Date.now()` to get the number of milliseconds since the Unix Epoch.
+# Prefer `Date.now()` to get the number of milliseconds since the Unix Epoch
 
-<!-- More detailed description. Remove this comment. -->
+[`Date.now()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now) is shorter and nicer.
 
 This rule is fixable.
 
 ## Fail
 
 ```js
-const foo = 'unicorn';
+const foo = new Date().getTime();
+```
+
+```js
+const foo = new Date().valueOf();
+```
+
+```js
+const foo = +new Date;
+```
+
+```js
+const foo = Number(new Date());
+```
+
+```js
+const foo = new Date() * 2;
 ```
 
 ## Pass
 
 ```js
-const foo = '🦄';
+const foo = Date.now();
+```
+
+```js
+const foo = Date.now() * 2;
 ```