Create third-party contributions file (#262)

A file containing details on our third-party contributors.

Author: chrispope

third-party contributors.md

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+## Third-Party Contributors
+
+The Boss Room sample game was created by Unity Technologies in partnership with external contributors: 
+
+[Storm Flag Games](https://stormflaggames.com)
+
+* Multiplayer Code
+* UI/UX
+* Icon Art
+* Environment Art
+* 3D Characters and Animations
+
+[Jason Hayes Music LLC](https://www.jasonhayesmusic.com/)
+
+* Music and Sound Effects
+
+[Room 8 Studio](https://room8studio.com/)
+
+* Visual FX
+
+Please familarise yourself with the [License](https://github.com/Unity-Technologies/com.unity.multiplayer.samples.coop/blob/master/LICENSE.md) type for these assets.
+You are free to use these assets in your own work. 
+
+Everyone at Unity including our third-party contributors is very excited to see what you do with them in your Unity projects!