Added at method in unordered maps

unordered_map/at.md

@@ -0,0 +1,36 @@
+# at
+
+**Description :** This function is used to modify values by using key of respective value.
+
+**Example** :
+
+```cpp
+// Demonstrates at()
+#include <iostream>
+#include <string>
+#include <unordered_map>
+
+int main(){
+    //Initializing a map with values as specified:
+    std::unordered_map<std::string, int> mymap = {
+        { "First", 100},
+        { "Second", 200},
+        { "Third", 300 } 
+    };
+
+    //Updating values with the help of at() method 
+    mymap.at("First") = 50;
+    mymap.at("Second") += 500;
+    mymap.at("Third") = mymap.at("First") + 950;
+
+    //Printing all three values
+    for (auto& x: mymap){
+        std::cout << x.first << ": " << x.second << std::endl;
+    }
+
+    return 0;
+}
+
+```
+
+**[Run Code](https://rextester.com/VEEQ37866)**