add postprocessor config example

docs/gallery-dl-example.conf

@@ -15,6 +15,26 @@
             "username": "foo",
             "password": "bar",
 
+            "postprocessors": [
+                {
+                    "name": "classify",
+                    "mapping": {
+                        "images": ["jpg", "png", "gif"],
+                        "ugoira": ["zip", "txt"]
+                    }
+                },
+                {
+                    "name": "zip",
+                    "keep-files": true,
+                    "compression": "store"
+                },
+                {
+                    "name": "exec",
+                    "async": false,
+                    "command": ["echo", "\n{user[account]} - {id}"]
+                }
+            ],
+
             "favorite":
             {
                 "directory": ["Pixiv", "Favorites", "{user[id]}"]
@@ -50,6 +70,15 @@
             "wait-max": 5.0
         },
 
+        "mangadex":
+        {
+            "postprocessors": [{
+                "name": "zip",
+                "keep-files": false,
+                "compression": "zip"
+            }]
+        },
+
         "flickr":
         {
             "access-token": "1234567890-abcdef",