docs: README for stack

Author: yeoshuheng

src/dataStructures/stack/README.md

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+# Stack
+Stack is a data structure that restricts 
+the order in which operations can be performed.
+
+![Stack data structure](https://cdn.programiz.com/sites/tutorial2program/files/stack.png)
+
+*Source: Programiz*
+
+### Operation Orders
+
+Stack follows a LIFO, last in first out order. 
+This means the most recent element 
+added to the stack is the one operations are conducted on first.
+
+A [queue](../queue/README.md) conducts operations in the opposite order.
+
+## Analysis
+
+As a stack only interacts with the most recent element regardless of operation,
+it keeps the pointer of the most recent element at hand, which is constantly updated as more 
+elements are removed / added. This allows stack operations to only incur a *O(1)* time complexity.
+
+## Notes
+
+### Stack vs Queues
+
+Stack and queues only differ in terms of operation order, you should aim to use a stack when
+you want the most recent elements to be operated on. 
+Some situations where a stack would work well include build redo / undo systems and backtracking problems.
+
+On the other hand, a queue allows you to operate on elements that enter first. Some situations where
+this would be useful include Breadth First Search algorithms and task / resource allocation systems.
+
+### Arrays vs Linked List
+It is worth noting that stacks can be implemented with either a array or with a [linked list](../linkedList/README.md).
+In the context of ordered operations, the lookup is only restricted to the first element.
+
+Hence, there is not much advantage in using a array, which only has a better lookup speed (*O(1)* time complexity) 
+to implement a stack. Especially when using a linked list to construct your stack
+would allow you to grow or shrink the stack as you wish.