readme: add evaluate snippet (#524)

* readme: add evaluate snippet

pretty important method that we don't have a prominent example for.

* feedback

* Update README.md

README.md

@@ -70,6 +70,32 @@ const puppeteer = require('puppeteer');
 
 See [`Page.pdf()`](https://github.com/GoogleChrome/puppeteer/blob/master/docs/api.md#pagepdfoptions) for more information about creating pdfs.
 
+**Example** - evaluate script in the context of the page
+
+```js
+const puppeteer = require('puppeteer');
+
+(async () => {
+  const browser = await puppeteer.launch();
+  const page = await browser.newPage();
+  await page.goto('https://example.com');
+  
+  // Get the "viewport" of the page, as reported by the page.
+  const dimensions = await page.evaluate(() => {
+    return {
+      width: document.documentElement.clientWidth,
+      height: document.documentElement.clientHeight,
+      deviceScaleFactor: window.devicePixelRatio
+    };
+  });
+  
+  console.log('Dimensions:', dimensions);
+
+  browser.close();
+})();
+
+See [`Page.evaluate()`](https://github.com/GoogleChrome/puppeteer/blob/master/docs/api.md#pageevaluatepagefunction-args) for more information on `evaluate` and related methods like `evaluateOnNewDocument` and `exposeFunction`.
+
 ## Default runtime settings
 
 **1. Uses Headless mode**