Update README.md

README.md

 ## Instructions:
 
 ### Building a basic `PhoneBook` class
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 <img src = "./PhoneBookUML.png">
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 * 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.