create Readme.md for stacks (TheAlgorithms#2279)

* create Readme.md for stacks

* Update README.md

Co-authored-by: Yang Libin <[email protected]>

Author: Amin-mashari

DataStructures/Stacks/README.md

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+# STACK
+
+stack is an ADT (abstract data type ) that act like list of objects but there is a diffrents.
+
+stack act is *LIFO* (Last In First Out), it means that when we want to get an element from the stack we get the last element in the stack.
+
+stack is bast on two methods ( functions) 
+
+## push & pop 
+
+**push**: add an alement to last index of stack. 
+
+for example: we have `1, 3, 5` in stack, then we call push(9),
+
+`9` will add to last index of stack -> `1, 3, 5 , 9`
+
+
+**pop**: remove the last element from stack.
+for example: we have `1, 3, 5 , 9` in stack, then we call pop(),
+
+the function will return `9` and the stack will change to `1, 3, 5`.