documentation for all major modules

ChatApp/app/src/main/java/client/UsernameSingleton.java

@@ -1,5 +1,8 @@
 package client;
 
+/**
+ * This class aims to make the username available to all controllers. (Recommended by JavaFX, because otherwise, FXMLLoader bugs and doesn't switch scenes properly.)
+ */
 public class UsernameSingleton {
     private static UsernameSingleton instance;
     private String username;
@@ -7,6 +10,10 @@ public class UsernameSingleton {
     private UsernameSingleton() {
     }
 
+    /**
+     * Get the instance of the singleton.
+     * @return the instance
+     */
     public static UsernameSingleton getInstance() {
         if (instance == null) {
             instance = new UsernameSingleton();