Add C example for recursion (#41)

03_recursion/c/01_countdown.c

@@ -0,0 +1,19 @@
+#include <stdio.h>
+
+void countdown(int i) {
+	printf("%d\n", i);
+	
+	// base case
+	if (i <= 0)
+		return;
+	//recursive case
+	else
+		countdown(i - 1);
+}
+
+int main(void) {
+
+	countdown(5);
+	
+	return 0;
+}