Update doubly linked list README.

Author: trekhleb

src/data-structures/doubly-linked-list/README.md

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 # Doubly Linked List
 
-In computer science, a doubly linked list is a linked data structure that consists of a set of sequentially linked records called nodes. Each node contains two fields, called links, that are references to the previous and to the next node in the sequence of nodes. The beginning and ending nodes' previous and next links, respectively, point to some kind of terminator, typically a sentinel node or null, to facilitate traversal of the list. If there is only one sentinel node, then the list is circularly linked via the sentinel node. It can be conceptualized as two singly linked lists formed from the same data items, but in opposite sequential orders.
+In computer science, a **doubly linked list** is a linked data structure that 
+consists of a set of sequentially linked records called nodes. Each node contains 
+two fields, called links, that are references to the previous and to the next 
+node in the sequence of nodes. The beginning and ending nodes' previous and next 
+links, respectively, point to some kind of terminator, typically a sentinel 
+node or null, to facilitate traversal of the list. If there is only one 
+sentinel node, then the list is circularly linked via the sentinel node. It can 
+be conceptualized as two singly linked lists formed from the same data items, 
+but in opposite sequential orders.
 
 ![Doubly Linked List](https://upload.wikimedia.org/wikipedia/commons/5/5e/Doubly-linked-list.svg)
 
+The two node links allow traversal of the list in either direction. While adding 
+or removing a node in a doubly linked list requires changing more links than the 
+same operations on a singly linked list, the operations are simpler and 
+potentially more efficient (for nodes other than first nodes) because there 
+is no need to keep track of the previous node during traversal or no need 
+to traverse the list to find the previous node, so that its link can be modified.
+
 ## References
 
 - [Wikipedia](https://en.wikipedia.org/wiki/Doubly_linked_list)
-- [YouTube](https://www.youtube.com/watch?v=JdQeNxWCguQ)
+- [YouTube](https://www.youtube.com/watch?v=JdQeNxWCguQ&t=7s&index=72&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)