Add Backdrop CMS (#1162)

Added Backdrop CMS to the open-source list + checked developer website and source code.

README.md

@@ -442,6 +442,7 @@ CMS are a practical way to setup a website with many features. CMS often come wi
 
   * [APIQ CMS](https://www.apiq.io/) - Simple and powerful Ruby on Rails CMS for developers. ([Demo](http://demo.apiq.io/), [Source Code](https://github.com/apiqcms/kms)) `MIT` `Ruby`
   * [Apostrophe 2](http://apostrophenow.org/) - A node.js CMS with a focus on extensible in-context editing tools. ([Demo](http://demo.apostrophenow.com/), [Source Code](https://github.com/punkave/apostrophe)) `MIT` `Nodejs`
+  * [Backdrop CMS](https://backdropcms.org/) - The comprehensive CMS for small to medium sized businesses and non-profits. ([Source Code](https://github.com/backdrop/backdrop)) `GPLv2` `PHP`
   * [Baun](https://bauncms.com/) - A modern, lightweight, extensible CMS for PHP. ([Source Code](https://github.com/BaunCMS/Baun)) `MIT` `PHP`
   * [BigTree CMS](https://www.bigtreecms.org/) - Straightforward, well documented, and capable written with PHP and MySQL. ([Source Code](https://github.com/bigtreecms/BigTree-CMS)) `LGPLv2` `PHP`
   * [CMS Made Simple](http://www.cmsmadesimple.org/) - Open source content management system, faster and easier management of website contents, scalable for small businesses to large corporations. ([Source Code](http://svn.cmsmadesimple.org/svn/cmsmadesimple/trunk/)) `GPL` `PHP`