Updated Developing extensions (markdown)

Developing-extensions.md

@@ -2,6 +2,7 @@ An extension is just a subdirectory in the `extensions` directory.
 
 Web ui interacts with installed extensions in the following way:
 
+- extension's `install.py` script, if it exists, is executed.
 - extension's scripts in the `scripts` directory are executed as if they were just usual user scripts, except:
   - `sys.path` is extended to include the extension directory, so you can import anything in it without worrying
   - you can use `scripts.basedir()` to get the current extension's directory (since user can name it anything he wants)
@@ -9,3 +10,13 @@ Web ui interacts with installed extensions in the following way:
 - extension's `style.css` file is added to the page
 
 For how to develop custom scripts, which usually will do most of extension's work, see [Developing custom scripts](Developing-custom-scripts).
+
+## install.py
+`install.py` is the script that is launched by the `launch.py`, the launcher, in a separate process before webui starts, and it's meant to install dependencies of the extension. The script is launched with `PYTHONPATH` environment variable set to webui's path, so you can just `import launch` and use its functionality:
+
+```python
+import launch
+
+if not launch.is_installed("aitextgen"):
+    launch.run_pip("install aitextgen==0.6.0", "requirements for MagicPrompt")
+```