Add minimum width to software requirments

[YetAnotherGerrit]

We decided that the new desktop UI will have a minimum with of 640px to be usable. Below that we recommend the mobile view instead.

This should be added to the client software requirments.

[ralf401]

I wonder if we explicitly should mention it... Isn't it already hard(er) to use when the width is below let's say 1280 / 1366?

In any case we should mention display scaling there too. Otherwise, people could complain like "My device hast 2000 pixels in width, but doesn't work" (e.g. because scaling is set to factor 4).

[YetAnotherGerrit]

Fore sure, 640px with will not be the same experience as a full-with desktop. But as part of the reponsive-epic we decided, it should at least be usable. If you don't think it should be in the documentation, it is fine. We just wanted to make sure, that the decision is accessable somewhere.