Add stackoverflow link regarding rand()

emper/Runtime.hpp

@@ -84,6 +84,7 @@ public:
 
 	Fiber* nextFiber();
 
+	// https://stackoverflow.com/a/3747462/194894
 	static inline int rand() {
 		seed = 214013 * seed + 2531011;
 		return (seed >> 16) & 0x7FFF;