refactoring: added comments

src/primzahl/primzahl.c

@@ -2,12 +2,15 @@
 #include <stdbool.h>
 
 bool primzahl(int num) {
+	//for the case zero or one
 	if (num <= 1)
 		return false;
 
+	//checking if there are any other divider
 	for (int i = 2; i < num; i++) {
 		if (num % i == 0)
 			return false;
 	}
+	//if none were found return true
 	return true;
 }