nuevo archivo USAGE.md

FossilOrigin-Name: dc0b361a17b7d939ead7bdbbf14a298a843eb9eb09e12f499330fb086503c3a8

USAGE.md

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+# uso de la libreria
+
+modelar la factura usando **facho/fe/form.py** ver **examples/generate-invoice-invoice.py(invoice)**, instanciar las extensiones requeridas ver **examples/generate-invoice-invoice.py(extensions)**
+una vez generado el objeto invoice y con las extensiones se procede a crear el XML, ejemplo:
+
+~~~python
+....
+validator = form.DianResolucion0001Validator()
+
+if not validator.validate(invoice):
+  for error in validator.errors:
+    raise print("ERROR:", error)
+  raise RuntimeError("invoice invalid")
+
+xml = form_xml.DIANInvoiceXML(invoice)
+extensions = module.extensions(invoice)
+for extension in extensions:
+  xml.add_extension(extension)
+
+form_xml.DIANWriteSigned(xml, "factura.xml", "llave privada", "frase")
+~~~