docs(api.md): add links to CDP guide and pptr.dev (#4159)

docs/api.md

@@ -3,6 +3,7 @@
 <!-- GEN:empty-if-release -->
 > NExt Release: **Mar 28, 2019**
 <!-- GEN:stop -->
+- Interactive Documentation: https://pptr.dev
 - API Translations: [中文|Chinese](https://zhaoqize.github.io/puppeteer-api-zh_CN/#/)
 - Releases per Chromium Version:
   * Chromium 73.0.3679.0 - [Puppeteer v1.12.2](https://github.com/GoogleChrome/puppeteer/blob/v1.12.2/docs/api.md)
@@ -3460,7 +3461,9 @@ The `CDPSession` instances are used to talk raw Chrome Devtools Protocol:
 - protocol methods can be called with `session.send` method.
 - protocol events can be subscribed to with `session.on` method.
 
-Documentation on DevTools Protocol can be found here: [DevTools Protocol Viewer](https://chromedevtools.github.io/devtools-protocol/).
+Useful links:
+- Documentation on DevTools Protocol can be found here: [DevTools Protocol Viewer](https://chromedevtools.github.io/devtools-protocol/).
+- Getting Started with DevTools Protocol: https://github.com/aslushnikov/getting-started-with-cdp/blob/master/README.md
 
 ```js
 const client = await page.target().createCDPSession();