[0x12] Update README.md

[0x12-6] Assembly program to print a message

[0x12-5_5] Demonstrate functionality

[0x12-5] Implement the print_before_main function

[0x12-4_4] Demonstrate functionality

[0x12-4] Implement the free_list function

[0x12-3_3] Demonstrate functionality

[0x12-3] Implement the add_node_end function

[0x12-2_2] Demonstrate functionality

[0x12-2] Implement the add_node function

[0x12-1_1] Demonstrate functionality

[0x12-1] Implement the list_len function

[0x12-0_0] Demonstrate functionality

[0x12-0] Implement the print_list function

[0x12] Add Singly linked lists