(new doc) add esdoc for the first method

.esdoc.json

@@ -0,0 +1,4 @@
+{
+  "source": "./src",
+  "destination": "./doc"
+}

.gitignore

@@ -1,4 +1,5 @@
 node_modules/
 coverage/
 dist/
+doc/
 .nyc_output/

package.json

@@ -17,6 +17,7 @@
     "babel-loader": "^7.1.1",
     "babel-preset-env": "^1.6.0",
     "chai": "^4.1.0",
+    "esdoc": "^0.5.2",
     "eslint": "3.19.0",
     "eslint-config-airbnb": "^15.0.2",
     "eslint-plugin-import": "2.6.1",

src/array/flatten.js

@@ -1,3 +1,13 @@
+/**
+ * Flattens `array` a single level deep.
+ *
+ * @param {Array} array The array to flatten.
+ * @returns {Array} Returns the new flattened array.
+ * @example
+ *
+ * flatten([1, [2, [3]])
+ * // => [1, 2, [3]]
+ */
 const flatten = array => [].concat(...array);
 
 export default flatten;