Add ATM challenge

atm_challenge.md

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+# ATM challenge
+
+## Introduction
+
+This challenge is to write a software for an ATM, where a few rules, must be applied in order to
+withdraw money.
+
+Source: https://dojopuzzles.com/problems/caixa-eletronico/
+
+> PS the source is in portuguese.
+
+## Exercise
+
+Develop a software that simulates a withdraw where the the user inputs the money amount and the
+software outputs is the money bills the user can take from the machine, following the rules:
+
+- The sofware needs to delivery as low money bills as possible.
+- Is only possible to withdraw valid amounts according to the available money bills.
+- There is no need to check the user's balance.
+- There is no need to count the number of available money bills.
+- The available money bills as $2, $5, $10, $20, $50.
+
+### Use case
+
+#### Case 1
+
+Given the user which wants to withdraw $30.
+
+The software needs to return one money bill for $20 and one for $10.
+
+#### Case 2
+
+Given the user which wants to withdraw $45.
+
+The software needs to return two money bill for $20 and one for $5.
+
+#### Case 3
+
+Given the user which wants to withdraw $21.
+
+The software needs to return an error since there is not available money bill to deliver $21.
+
+### Global Requirements
+
+- The challenge can be develop using any programming language.
+- No external external service and library needs to be used
+- The solution must have a way to execute, CLI, file or any other interaction.
+- Tests are always welcome
+- Create a README file with instructions on how to run the application and the tests.