Add 'call for testers' to readme

Created by: elbenfreund

One of the biggest issue developing this extension is a lack of dedicated testing. As it became obvious that automated unit tests are not going to happen on any relevant scale, we really need manual testers.

As I personally do not really use this extension I am stuck with just some very basic tests on a virtual box installation.

This leaves me with two shitty options:

• merge new code without any actual test feedback
• wait until the reporter of an issue may or may not give it a try

In order to improve this situation I would be good to add a call for help to the readme, asking for people willing to try out new feature branches in order to speed up the release process.