Update README.md

as for singly linked list and doubly linked list, here is something different
to reverse, we need one additional pointer for singlyLL to record the influence of time,cause we cannot access the predecessor directly.
it is seen in the perspective of time 
as for singly, we keep two pointers, one referring to the previous item, one for the current
          node* next;
          next = current->next;
   then   current->next = previous;
          previous = current;
          current = next;
we use the previous to remain the node in the past in the perspective of time

as for doubly,
we only need 1 for current, and another one as temp(for swapping)
temp = current-> next;
current->next = current->prev;
current->prev = temp;

current = current-> prev; // attention on this