Update README

Author: gabino

README.md

@@ -13,20 +13,66 @@ Python client library for CEP (http://www.banxico.org.mx/cep/)
 pip install cepmex
 ```
 
-### Uso
+## Development & Testing
+
+You can use a staging environment to test the library:
 
 ```python
-from datetime import date
+from cep import Config
+
+Config.BASE_URL = 'http://www.banxico.org.mx/cep-beta'
+```
+
+To run unit tests, use `pytest`.
+```bash
+pytest
+```
+
+### Usage
 
+```python
+from datetime import date
 from cep import Transferencia
+from cep.exc import NotFoundError
 
-tr = Transferencia.validar(
-    fecha=date(2019, 4, 12),
-    clave_rastreo='CUENCA1555093850',
-    emisor='90646',  # STP
-    receptor='40012',  # BBVA
-    cuenta='012180004643051249',
-    monto=8.17,
-)
-pdf = tr.descargar()
+try:
+    tr = Transferencia.validar(
+        fecha=date(2019, 4, 12),
+        clave_rastreo='CUENCA1555093850',
+        emisor='90646',  # STP
+        receptor='40012',  # BBVA
+        cuenta='012180004643051249',
+        monto=8.17,
+    )
+    pdf = tr.descargar()
+    with open('CUENCA1555093850.pdf', 'wb') as f:
+        f.write(pdf)
+except NotFoundError as e:
+    print('No se encontro la transferencia')
+```
+
+## Validate Transfer Parameters
+
+Use the `validar` method to validate a transfer with the following parameters:
+
+### Required Parameters:
+- `fecha` (`datetime.date`): Transfer date.
+- `clave_rastreo` (`str`): Transfer tracking key.
+- `emisor` (`str`): Transfer sender bank code.
+- `receptor` (`str`): Transfer receiver bank code.
+- `cuenta` (`str`): Transfer account number.
+- `monto` (`Decimal`): Transfer amount.
+
+### Optional Parameters:
+- `pago_a_banco` (`bool`, default=`False`): Set to `True` for transfer types 4 and 31.
+
+## Download Transfer Data
+
+Use the `descargar` method to download a transfer in one of the following formats:
+- `PDF` (default)
+- `XML`
+- `ZIP`
+
+```python
+tr.descargar(formato='XML')
 ```