Add a readme

There hasn't been a README this whole time... I guess there hasn't been
anything I need to immediately communicate to someone looking at the
source repository.

There is now a README so I have somewhere to record the extra licensing
information (for the Kenney asset).

README.md

@@ -0,0 +1,14 @@
+# Asteroids
+
+*Another* Asteroids game I'm making. This time in Rust with the Bevy game engine.
+
+## License
+
+| File(s) | License |
+|-|-|
+| * | AGPLv3 |
+| assets/* | CC0 |
+
+(the most-specific match is the applicable license)
+
+The sound files are from KenneyNL's "Sci-Fi Sounds (1.0)" pack. Find their work at [www.kenney.nl].