List celery as an optional dependency

Author: Hugo Osvaldo Barrera

README.rst

@@ -39,6 +39,11 @@ Installation should generally be done via pip::
 
     pip install django-mercadopago
 
+If you'll be using the asynchronous processing, celery is required. You'll need
+to set up celery itself for your project as well, obviously::
+
+    pip install django-mercadopago[celery]
+
 Configuration
 -------------
 

setup.py

@@ -16,6 +16,7 @@
     setup_requires=['setuptools_scm'],
     extras_require={
         'fixtures': ['factory-boy'],
+        'celery-task': ['celery'],
     },
     classifiers=[
         'Development Status :: 4 - Beta',