Document the creation of the mirrors folder

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

README.rst

@@ -15,6 +15,11 @@ Run this locally
 * The ``mirror_from_pagure_bus.py`` is a fedora-messaging consumer.
   To run it you will have to follow these steps (for dev):
 
+  * Create the folder where mirrors are stored by default:
+    ::
+
+        mkdir mirrors
+
   * Adjust the example configuration file:
     ::
 
@@ -23,6 +28,7 @@ Run this locally
 
   * Run the consumer:
     ::
+
         PYTHONPATH=. fedora-messaging --conf my_config.toml consume