Add extracted source directory and README navigation

extracted-source/node_modules/axios/lib/helpers/callbackify.js

@@ -0,0 +1,18 @@
+import utils from '../utils.js';
+
+const callbackify = (fn, reducer) => {
+  return utils.isAsyncFn(fn)
+    ? function (...args) {
+        const cb = args.pop();
+        fn.apply(this, args).then((value) => {
+          try {
+            reducer ? cb(null, ...reducer(value)) : cb(null, value);
+          } catch (err) {
+            cb(err);
+          }
+        }, cb);
+      }
+    : fn;
+};
+
+export default callbackify;