Added notes around the PnP JS Core

Author: VesaJuvonen

docs/spfx/tools-and-libraries.md

@@ -24,6 +24,8 @@ You can choose any one of a number of JavaScript frameworks to develop client-si
 
 Because client-side web parts are components that are dropped into a SharePoint page, we recommend that you choose a JavaScript framework that supports a similar component model. Lightweight frameworks such as React, Handlebars, and Angular 2 all support a component model and are well suited to building client-side web parts. 
 
+We also recommend you to have a look on [SharePoint PnP JavaScript Core library](https://github.com/OfficeDev/PnP-js-core), which is a community driven effort targeted for providing easy access on SharePoint REST APIs. 
+
 ## Node Package Manager (npm)
 
 SharePoint client-side development tools use the [npm](https://www.npmjs.com/) package manager, which is similar to [NuGet](https://www.nuget.org/), to manage dependencies and other required JavaScript helpers. npm is typically included as part of Node.js setup.