docs: Add source module readme

modules/source/README.md

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+# Source Engine Module
+
+The Source game management plugin for Mumo can dynamically move players into on-the-fly created channel structures representing in-game team setup.
+This is achieved by using data gathered from Mumble's Positional-Audio and does not require cooperation by the game server.
+
+The following source engine based games are supported:
+
+* Team Fortress 2
+* Day of Defeat: Source
+* Counter-Strike: Source
+* Half-Life 2: Deathmatch
+
+## Enabling the `source` module
+
+1. Link or copy the ''source.ini'' file from the `modules-available` folder into the `modules-enabled` folder\
+  `cd modules-enabled`\
+  `ln -s ../modules-available/source.ini`
+2. Check whether the defaults in `source.ini` are ok for your setup. They should be sane for basic setups.
+3. Restart mumo