Update 'README.md'

README.md

@@ -19,9 +19,12 @@
 
 ![PhoneBookDesign](./PhoneBookUML.png)
 
+In this diagram, C stands for Class, f stnads for field, and m stands for method.
+So, it can give you a specification for the class and the things it needs to do.
+
 * Create a `PhoneBook` class that holds names and phone numbers.
 * You can use an [associative data type](https://en.wikipedia.org/wiki/Associative_array) (one which stores items as keys paired with values).
-* **Hint:** You should use a sorted map.
+* **Hint:** You should use a sorted map from the java standard library.
 
 
 * Your PhoneBook class should have the following method