Add info for new mailing list to documentation.

[bjoernbroetz]

This adds the info about our new user mailing list to the documentation.

Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• Give this pull request a descriptive title that can be used as a one line summary in a changelog
• Make sure your code is composed of functions of no more than 50 lines and uses meaningful names for variables
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Preferably Codacy code quality checks pass, however a few remaining hard to solve Codacy issues are still acceptable. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.

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Closes #1606

[bjoernbroetz]

Let's keep this on hold till we sort out the discussion of today's telco with @bouweandela and the others.

[bjoernbroetz]

I also deactivated the subscribtion of this list. It's now on invitation only. Then the German-Issue is gone.

[bjoernbroetz]

I also deactivated the subscribtion of this list. It's now on invitation only. Then the German-Issue is gone.

Ok, now that it is merged I activated again the subscription as described in the docu now.