Credits.md

(very) Incomplete list of contributors and the assets they created:

• AEfremov: shared/textures/objects/berserkerkit.png, shared/textures/objects/flamerkit.png, shared/textures/objects/predatorkit.png, shared/textures/objects/medikit.png
• pewpew (@BranDougherty): shared/interface-gfx/friend.png, shared/interface-gfx/mic.png, shared/interface-gfx/connection.png