Fixes #326 - Added documentation for linked lists (#327)

Author: itchylol742

Data Structures/Linked Lists/Doubly-linked-list.png

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+# Linked list
+
+A linked list is a collection of nodes which are connected to each other by pointers. A singly linked list has a pointer to the first node, and each node has a pointer to the next node. A double linked list has a pointer to the first and last node, and each node has a pointer to the next and previous node. Each node also contains data.
+
+Linked lists have a runtime of O(1) when inserting or reading data from the front of the list (and back for doubly linked lists), but have a runtime of O(n) when accessing a specific element in the list or searching the list.
+
+Images from Wikipedia under public domain.
+
+# Singly linked list diagram:
+![alt text](Singly-linked-list.png)
+# Doubly linked list diagram:
+![alt text](Doubly-linked-list.png)