add note about Piston

NOTES.md

@@ -1,5 +1,19 @@
 # Gammarena Notes
 ## Engine
+### Piston
+https://github.com/PistonDevelopers/piston
+
+#### Pros
+* Pure Rust
+* Under active development
+* Probably the most popular Rust game engine
+* [Widely used](https://github.com/PistonDevelopers/piston/wiki/Games-Made-With-Piston)
+* Has [tutorials](https://github.com/pistondevelopers/piston-tutorials) and [examples](https://github.com/pistondevelopers/piston-examples)
+* Also has [documentation](https://github.com/PistonDevelopers/piston/blob/master/GUIDE.md)
+
+#### Cons
+* May be lacking in features we require
+
 ### unrust
 https://github.com/edwin0cheng/unrust