docs: adding multiple apps story (#5737)

Fixes: #5726

Author: sumitarora

docs/documentation/stories/multiple-apps.md

@@ -0,0 +1,66 @@
+# Multiple Apps integration
+
+Angular CLI support multiple applications within one project.
+You use the `apps` array in `.angular-cli.json` to list files and folders you want to use for different apps.
+
+By default one app is created when then new project is created and `apps` array looks like:
+```
+"apps": [
+  {
+    "root": "src",
+    ...
+    "main": "main.ts",
+    "polyfills": "polyfills.ts",
+    "test": "test.ts",
+    "tsconfig": "tsconfig.app.json",
+    "testTsconfig": "tsconfig.spec.json",
+    "prefix": "app",
+    ...
+  }
+],
+```
+
+To create another app you can copy the app object and then change the values for the options you want to change. eg. If I want to create another app with different `main`, `polyfills`, `test` and `prefix` and keep other configurations such as `assets`, `styles`, `environment` etc. same. I can add it to apps array as below.
+```
+"apps": [
+  {
+    "root": "src",
+    ...
+    "main": "main.ts",
+    "polyfills": "polyfills.ts",
+    "test": "test.ts",
+    "tsconfig": "tsconfig.app.json",
+    "testTsconfig": "tsconfig.spec.json",
+    "prefix": "app",
+    ...
+  },
+  {
+    "root": "src",
+    ...
+    "main": "main2.ts",
+    "polyfills": "polyfills2.ts",
+    "test": "test2.ts",
+    "tsconfig": "tsconfig.app.json",
+    "testTsconfig": "tsconfig.spec.json",
+    "prefix": "app2",
+    ...
+  }  
+],
+```
+Now we can `serve`, `build` etc. both the apps by passing the app index with the commands. By default, it will pick the first app only.
+
+To serve the first app: `ng serve --app=0` or `ng serve --app 0`
+
+To serve the second app: `ng serve --app=1` or `ng serve --app 1`
+
+You can also add the `name` property to the app object in `apps` array and then pass it to commands to distinguish between different applications.
+```
+"apps": [
+  {
+    "name": "app1",
+    "root": "src",
+    "outDir": "dist",
+....
+```
+To serve application by name `ng serve --app=app1` or `ng serve --app app1`.
+