Improvement on email client function

In the section "Functions should do one thing" this change uses
the Array.filter method to get the active clients, then emailing
those with the forEach loop.

Author: rodu

README.md

@@ -274,15 +274,9 @@ function emailClients(clients) {
 **Good**:
 ```javascript
 function emailClients(clients) {
-  clients.forEach(client => {
-    emailClientIfNeeded(client);
-  });
-}
-
-function emailClientIfNeeded(client) {
-  if (isClientActive(client)) {
-    email(client);
-  }
+  clients
+    .filter(isClientActive)
+    .forEach(email);
 }
 
 function isClientActive(client) {