Add info about Plone 6 Docker images

Author: avoinea

plone/content.md

@@ -6,7 +6,8 @@
 
 ## Features
 
--	Images for Plone 5.x and Plone 4.x
+-	Images for **Plone 5.x** and **Plone 4.x**
+-	Images for **Plone 6.x** are available at [plone/plone-backend](https://hub.docker.com/r/plone/plone-backend) and [plone/plone-frontend](https://hub.docker.com/r/plone/plone-frontend)
 -	Enable add-ons via environment variables
 -	Choose between [Debian](https://www.debian.org/) or [Alpine](http://www.alpinelinux.org/) based images.
 -	Built-in RelStorage support, configurable via environment variables (requires Plone 5.2.4+)