[docs] adding CONTRIBUTING document skeleton

CONTRIBUTING.md

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+# Welcome to Northstar contribution guide
+
+Thank you for investing your time in contributing to our project!
+
+## Repositories architecture
+
+Northstar software is splitted into several repositories:
+
+* [Northstar](https://github.com/R2Northstar/Northstar) is the main repository; it only contains downloadable 
+versions of the client and wiki documentation (it does not contain source code).
+* [NorthstarMasterServer](https://github.com/R2Northstar/NorthstarMasterServer)
+holds source code of the master server, which is responsible for authentication, persistence, custom servers
+listing.
+* [NorthstarLauncher](https://github.com/R2Northstar/NorthstarLauncher) holds source code responsible for 
+injecting custom content (mods) into original game files.
+* [NorthstarMods](https://github.com/R2Northstar/NorthstarMods) contains mods recreating server gamelogic,
+enabling custom serveurs hosting.
+
+## Issues
+
+### Create a new issue 
+
+If you find a bug while using Northstar, before posting it, please ensure that a corresponding issue does not 
+already exists.
+
+Also, please double-check that you're opening an issue in the correct repository (read carefully `Repositories architecture`
+of this document if you're not sure).
+