make min max average and sum snippets

snippets/sum.md

@@ -0,0 +1,13 @@
+### sum
+
+Returns the sum of an of two or more numbers/arrays.
+
+Use `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`.
+
+```js
+const sum = (...arr) => [].concat(...arr).reduce((acc, val) => acc + val, 0);
+```
+
+```js
+sum([1, 2, 3, 4]); // 10
+```