improved docs

physics/collision/matrix.go

@@ -60,4 +60,4 @@ func (m *Matrix) Get(i, j int) bool {
 func (m *Matrix) Reset() {
 
 	*m = make([][]bool, 0)
-}
+}

physics/doc.go

@@ -3,5 +3,6 @@
 // license that can be found in the LICENSE file.
 
 // Package physics implements a basic physics engine.
+// WARNING: This package is experimental and incomplete!
 package physics