add triangular number sequence task`

Languages/js-rush/task5/main.js

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+function main() {
+    console.log(triangle(1))
+    console.log(triangle(6))
+    console.log(triangle(215))
+}
+
+function triangle(num) {
+    var result = 1
+    for (var i = 2; i <= num; ++i)
+        result += i
+    return result
+}
+
+main()

Languages/js-rush/task5/readme.md

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+# Triangular Number Sequence
+
+This Triangular Number Sequence is generated from a pattern of dots that form a triangle. The first 5 numbers of the sequence, or dots, are:
+```
+1, 3, 6, 10, 15
+```
+This means that the first triangle has just one dot, the second one has three dots, the third one has 6 dots and so on.
+
+Write a function that returns the number of dots when given its corresponding triangle number of the sequence.
+
+### Examples
+```js
+triangle(1) ➞ 1
+triangle(6) ➞ 21
+triangle(215) ➞ 23220
+```