feat: should be able to see the balance (#34)

* feat: should be able to see the balance
* Update README.md

Co-authored-by: Julia Possamai <jpossamai@coderockr.com>

Author: lucassabreu

README.md

@@ -11,12 +11,12 @@ In this challenge you should build an API for an application that stores and man
     2. An investment should not be or become negative.
 2. __View__ of an investment with its initial amount and expected balance.
     1. Expected balance should be the sum of the invested amount and the [gains][].
+    2. If an investment was already withdrawn then the balance must reflect the gains of that investment
 3. __Withdrawal__ of a investment.
     1. The withdraw will always be the sum of the initial amount and its gains,
        partial withdrawn is not supported.
     2. Withdrawals can happen in the past or today, but can't happen before the investment creation or the future.
-    3. [Taxes][taxes] need to be applied to the withdrawals before showing the
-       final value.
+    3. [Taxes][taxes] need to be applied to the withdrawals before showing the final value.
 4. __List__ of a person's investments
     1. This list should have pagination.