Update app-manifest.md (#521)

Adding a line about where to assign the app manifest, as I was not able to find this mentioned anywere.

docs/en/manuals/app-manifest.md

@@ -12,6 +12,9 @@ Also application manifest contains some options to control compiling code for HT
 
 ![](images/app_manifest/app-manifest.png)
 
+# Applying the manifest
+
+In `game.project`, assign the manifest to `Native Extensions` -> `App Manifest`.
 
 ## Physics
 
@@ -106,4 +109,4 @@ That options relates to `html5.heap_size` in game.project [link](https://defold.
 YAML field name: **stackSize**
 Default value: **5242880**
 
-The stack size of application. For more information look [link](https://emscripten.org/docs/tools_reference/settings_reference.html?highlight=environment#stack-size). Value in bytes.
+The stack size of application. For more information look [link](https://emscripten.org/docs/tools_reference/settings_reference.html?highlight=environment#stack-size). Value in bytes.