doc: documentation added

docs/ilc_app_interface.md

@@ -16,7 +16,7 @@ ILC also supports apps that have client side rendering only.
 During the course of a single-spa page, registered applications are loaded, bootstrapped (initialized), mounted, unmounted, and unloaded. 
 ILC (with the help of the [single-spa](https://single-spa.js.org/)) provides hooks into each phase via `lifecycles`.
 
-See more information about the [lifecycle functions here](https://single-spa.js.org/docs/building-applications#lifecyle-props).
+See more information about the [lifecycle functions here](https://single-spa.js.org/docs/building-applications/#lifecyle-props).
 
 ### Custom props that are passed to every app
 
@@ -25,3 +25,27 @@ See more information about the [lifecycle functions here](https://single-spa.js.
 * `getCurrentBasePath(): string` - returns same value as `basePath` param in `routerProps` query parameter
 * `errorHandler(error, errorInfo = {}): void` - app MUST use it to propagate all unhandled errors
 * `appId` - Unique application ID, if same app will be rendered twice on a page - it will get different IDs
+
+
+### Init code during app bundle loading
+
+Sometimes you need to run some initialization code right after app bundle will be loaded in the browser and usually you 
+want to be able to pass some configuration properties to that code.
+
+ILC allows you to export a function called `mainSpa(props)` that will receive application properties that were defined in 
+_Registry_ in it's first argument. 
+This function should return an object with "single-spa" [lifecycle functions](https://single-spa.js.org/docs/building-applications/#lifecyle-props).
+
+**Example of possible use case:**
+```javascript
+// File specified as Webpack entry point
+export const mainSpa = (props) => {
+	if (props.publicPath) {
+		__webpack_public_path__ = props.publicPath;
+	} else {
+		console.warn(`Can't determine value of the "__webpack_public_path__", falling back to default one...`);
+	}
+
+	return require('./app-bootstrap'); // Returns: {bootstrap: () => {}, mount: () => {}, unmount: () => {}}
+};
+```