Added usage example in the readme

Author: Bernardo-MG

README.rst

@@ -83,6 +83,21 @@ Usage
 The application has been coded in Python, without using any particular
 framework.
 
+To use it just import the parser:
+
+    >>> from dice_notation.parser import DiceParser
+
+And then use it to parser a dice notation expression:
+
+    >>> parser = DiceParser()
+    >>> dice = parser.parse('1d6+2')
+
+The result can be accessed just by calling the 'roll' method as many times as
+needed, which will generate a new random value each time it is called.
+
+    >>> print(dice.roll())
+    >>> print(dice.roll())
+
 Collaborate
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