Updated Developing extensions (markdown)

Developing-extensions.md

@@ -9,6 +9,12 @@ Web ui interacts with installed extensions in the following way:
 - extension's javascript files in the `javascript` directory are added to the page
 - extension's localization files in the `localizations` directory are added to settings; if there are two localizations with same name, they are not merged, one replaces another.
 - extension's `style.css` file is added to the page
+- if extension has `preload.py` file in its root directory, it is loaded before parsing commandline args
+- if extension's `preload.py` has a preload function, it is called, and commandline args parser is passed to it as an argument. Here's an example of how to use it to add a command line argument:
+```python
+def preload(parser):
+    parser.add_argument("--wildcards-dir", type=str, help="directory with wildcards", default=None)
+```
 
 For how to develop custom scripts, which usually will do most of extension's work, see [Developing custom scripts](Developing-custom-scripts).