Update README.md

Add basic documentation explaining the `nconf.required` method

README.md

@@ -149,6 +149,18 @@ Removes the store with the specified `name.` The configuration stored at that le
 
 ``` js
   nconf.remove('file');
+```y
+
+### nconf.required(keys)
+Declares a set of string keys to be mandatory, and throw an error if any are missing.
+
+```js
+  nconf.defaults({
+    keya: 'a',
+  });
+
+  nconf.required(['keya', 'keyb']);
+  // Error: Missing required keys: keyb
 ```
 
 ## Storage Engines