add digital-distribution section

and introduce Desurium as an alternative to Steam

Desurium is a free community-driven, open-source client for the desura service and licensed under GPL-3.
Desurium is based on the official client which is called "desura". Desuras source code was published in november 2011 and it hasn't changed since then (unlike Desurium).
There has been some licensing discussion recently: https://github.com/desura/Desurium/issues/509

This is of course only the _client_. The service where you actually buy the stuff is run by 'DesuraNET Pty Ltd.', but it has been acquired by 'Linden Lab' recently, see https://en.wikipedia.org/wiki/Desura#Acquisition_by_Linden_Lab

The servers are proprietary and we should discuss that matter. However, the client being free is at least one step forward compared to steam. Also: unlike steam... you actually own the games you buy.