Update README.md

Author: Jenni-Foued

0x13-more_singly_linked_lists/README.md

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-# More singly linked lists
+# <div align="center">0x13. C - More singly linked lists</div>
+
+## About
+
+A follow up project on [singly linked lists](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x12-singly_linked_lists)
+
+## Files & Descriptions
+
+[0-print_listint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/0-print_listint.c)  a function that prints all the elements of a `listint_t` list
+
+[1-listint_len.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/1-listint_len.c)  a function that returns the number of elements in a linked `listint_t` list
+
+[2-add_nodeint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/2-add_nodeint.c)  a function that adds a new node at the beginning of a `listint_t` list
+
+[3-add_nodeint_end.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/3-add_nodeint_end.c)  a function that adds a new node at the end of a `listint_t` list
+
+[4-free_listint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/4-free_listint.c)  a function that frees a `listint_t` list
+
+[5-free_listint2.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/5-free_listint2.c)   a function that frees a `listint_t` list
+
+[6-pop_listint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/6-pop_listint.c)  a function that deletes the head node of a `listint_t` linked list, and returns the head node’s data (n)
+
+[7-get_nodeint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/7-get_nodeint.c)  a function that returns the nth node of a `listint_t` linked list
+
+[8-sum_listint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/8-sum_listint.c)  a function that returns the sum of all the data (n) of a `listint_t` linked list
+
+[9-insert_nodeint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/9-insert_nodeint.c)  a function that inserts a new node at a given position
+
+[10-delete_nodeint.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x13-more_singly_linked_lists/10-delete_nodeint.c)  a function that deletes the node at index `index` of a `listint_t` linked list