[docs] add kemono text content example (#2963)

docs/gallery-dl-example.conf

@@ -127,6 +127,23 @@
             ]
         },
 
+        "kemonoparty": {
+            "postprocessors": [
+                {
+                    "name": "metadata",
+                    "event": "post",
+                    "filename": "{id} {title}.txt",
+
+                    "#": "write text content and external URLs",
+                    "mode": "custom",
+                    "format": "{content}\n{embed[url]:?/\n/}",
+
+                    "#": "onlx write file if there is an external link present",
+                    "filter": "embed.get('url') or re.search(r'(?i)(gigafile|xgf|1drv|mediafire|mega|google|drive)', content)"
+                }
+            ]
+        },
+
         "flickr":
         {
             "access-token": "1234567890-abcdef",