Added accumulate() method of vectors

snippets/vector/accumulate.cpp

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+// Style Guide => https://github.com/Bhupesh-V/30-Seconds-Of-STL/blob/master/CONTRIBUTING.md/#Style Guide
+/*
+    Author : Deepanshu Midha
+    Date   : 01/10/2022
+    Time   : 05:50
+    Description : Returns the sum of all the values lying in a range between [first, last)
+*/
+
+#include <bits/stdc++.h>
+#include <vector>
+#include<numeric>
+
+int main() {
+    std::vector<int> v = {3, 13, 27};
+    int sum = 7;
+    std::cout << "Result using accumulate: ";
+    std::cout << accumulate(v.begin(), v.end(), sum);
+
+	return 0;
+}

vector/accumulate.md

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+# Accumulate
+
+**Description :** 
+- This function returns the sum of all the values lying in a range between [first, last) with the variable sum. 
+- Usually the sum of elements in a particular range or a complete array is found using a linear operation which requires adding all the elements in the range one by one and storing it into some variable after each iteration.
+
+**Example** :
+
+```cpp
+#include <bits/stdc++.h>
+#include <vector>
+#include<numeric>
+
+int main() {
+    std::vector<int> v = {3, 13, 27};
+    int sum = 7;
+    std::cout << "Result using accumulate: ";
+    std::cout << accumulate(v.begin(), v.end(), sum);
+
+	return 0;
+}
+```
+**[See Sample code](https://github.com/deepanshumidha5140/30-seconds-of-cpp/blob/master/snippets/vector/accumulate.cpp)**
+**[Run Code](https://rextester.com/BFYS36316)**