add documentation for priority queue (#213)

docs/Priority-Queue/index.md

@@ -0,0 +1,207 @@
+# Priority Queue 
+The ``` PriorityQueue()  ``` function returns an object upon which various priority queue based operations can be performed.
+
+### Syntax  
+> new M.PriorityQueue()  
+
+### Arguments  
+> None  
+    
+### Return  
+> Object
+    
+### Examples
+```js
+const pQueue = new M.PriorityQueue();
+console.log(pQueue);   // PriorityQueue  { queueArr: [] }
+```
+### Info:
+Implemented using JavaScript `class`.
+
+### Methods
+- queue() - This method prints all the elements of the Priority queue in FIFO order.
+- push() - This method adds an element to the Priority queue according to its priority.
+- pop() - This method removes an element from the Priority queue.
+- front() - This method returns the front element of the Priority queue.
+- rear() - This method returns the last element of the Priority queue.
+- empty() - This method returns true if the Priority queue is empty.
+- size() - This method returns the size of the Priority queue.
+
+------
+#### push() function
+
+##### Syntax
+> pQueue.push(value)
+
+##### Arguments
+> Number
+
+##### Return
+> Boolean true
+
+##### Examples  
+- Valid:  
+    ```js
+    const pQueue = new M.PriorityQueue(); 
+    pQueue.push(10);
+    pQueue.push(20);
+    ```  
+- Invalid:  
+    ```js
+    /*
+     * TypeError: Invalid argument received: <argument>
+     * 'PriorityQueue.push()' only accept a number!
+     */
+    pQueue.push("70");
+    pQueue.push([2,45]);
+    pQueue.push("five");
+    ```
+
+##### Info:
+Implemented using JavaScript built-in `push()` function.
+
+------
+#### pop() function
+
+##### Syntax
+> pQueue.pop()
+
+##### Arguments
+> None
+
+##### Return
+> Number
+
+##### Examples
+```js
+const pQueue=new M.PriorityQueue();
+pQueue.push(10);
+pQueue.push(20);
+pQueue.push(30);
+console.log(pQueue.pop())   // 10
+```
+
+##### Info
+Implemented using JavaScript built-in `pop()` function.
+
+------
+#### front() function
+
+##### Syntax
+> pQueue.front()
+
+##### Arguments
+> None
+
+##### Return
+> Number
+
+##### Examples
+```js
+const pQueue=new M.PriorityQueue();
+pQueue.push(10);
+pQueue.push(20);
+pQueue.push(30);
+console.log(pQueue.front());   // 10
+```
+
+##### Info
+Implemented using JavaScript `if()` condition.
+
+------
+
+#### rear() function
+
+##### Syntax
+> pQueue.rear()
+
+##### Arguments
+> None
+
+##### Return
+> Number
+
+##### Examples
+```js
+const pQueue=new M.PriorityQueue();
+pQueue.push(10);
+pQueue.push(20);
+pQueue.push(30);
+console.log(pQueue.rear());   // 30
+```
+
+##### Info
+Implemented using JavaScript `if()` condition.
+
+------
+
+#### size() function
+##### Syntax
+> pQueue.size()
+
+##### Arguments
+> None
+
+##### Return
+> Number
+
+##### Examples
+```js
+const pQueue=new M.PriorityQueue();
+pQueue.push(10);
+pQueue.push(20);
+pQueue.push(30);
+console.log(pQueue.size());   // 3
+```
+
+##### Info
+Implemented using JavaScript `length` property.
+
+------
+
+#### empty() function
+##### Syntax
+> pQueue.empty()
+
+##### Arguments
+> None
+
+##### Return
+> Boolean
+
+##### Examples
+```js
+const pQueue=new M.PriorityQueue();
+pQueue.push(10);
+pQueue.push(20);
+console.log(pQueue.empty());   // False
+```
+
+##### Info
+Implemented using JavaScript `length` property and `ternary` operator.
+
+------
+
+#### queue() function
+##### Syntax
+> pQueue.queue()
+
+##### Arguments
+> None
+
+##### Return
+> String
+
+##### Examples
+```js
+const pQueue=new M.PriorityQueue();
+pQueue.push(10);
+pQueue.push(20)
+pQueue.push(30)
+console.log(pQueue.queue());   // 10 20 30
+```
+
+##### Info
+Implemented using JavaScript built-in `join` function.
+
+------