added developer warning to vagrant & docker install

contributing-install-docker.rst

@@ -3,6 +3,8 @@ Install with Docker
 
 Docker is a container-based software framework for automating deployment of applications. Our Docker image is a **single container** based application designed to have Zammad **up and running fast for testing proposes**.
 
+Be aware that Vagrant is meaned for developers and therefore uses our unstable packages from the "develop" branch on GitHub.
+
 Your Docker environment needs to be up and running.
 
 You can find the image at https://hub.docker.com/r/zammad/zammad/

contributing-install-vagrant.rst

@@ -3,6 +3,8 @@ Install with Vagrant
 
 Vagrant is a tool for building complete development environments. With an easy-to-use workflow and focus on automation, Vagrant lowers development environment setup time, increases development/production parity, and makes the "works on my machine" excuse a relic of the past.
 
+Be aware that Vagrant is meaned for developers and therefore uses our unstable packages from the "develop" branch on GitHub.
+
 Let’s begin using Vagrant!
 First be sure that a Vagrant provider is installed. You can use "Virtual Box" from https://www.virtualbox.org .
 
@@ -38,3 +40,24 @@ After "vagrant up"
 ::
 
  vagrant ssh
+
+
+Problems to start the VM?
+=========================
+
+Iy you get errors like:
+
+::
+
+ A VirtualBox machine with the name 'Zammad' already exists.
+ Please use another name or delete the machine with the existing
+ name, and try again.
+
+
+Use the following commands to fix it:
+
+::
+
+ vboxmanage controlvm Zammad poweroff
+ vboxmanage unregistervm Zammad
+