doc: add badges & improve PR guidelines

Add npm version & license badges.
Add the makeapullrequest.com badge and link
in CONTRIBUTING.md.

README.md

@@ -1,5 +1,10 @@
 ![signal-k-logo-tagline](https://user-images.githubusercontent.com/1049678/34687576-d3e9af3e-f4b7-11e7-875e-80fce5a6e62b.png)
 
+[![npm version](https://badge.fury.io/js/signalk-server.svg)](https://badge.fury.io/js/signalk-server)
+[![npm license](https://img.shields.io/npm/l/signalk-server.svg)](https://www.npmjs.com/package/signalk-server)
+[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com)
+
+
 
 An implementation of a [Signal K](http://signalk.org) server in Node.js. Intended to run on embedded devices (e.g. Raspberry Pi, Beaglebone or UDOO).
 

docs/CONTRIBUTING.md

@@ -1,3 +1,5 @@
+**Working on your first Pull Request?** You can learn how from this *free* series [How to Contribute to an Open Source Project on GitHub](https://egghead.io/series/how-to-contribute-to-an-open-source-project-on-github)
+
 ###  Submitting a Pull Request (PR)
 Before you submit your Pull Request (PR) consider the following guidelines: