fix(docs): Make clear that the app directory has to be named like the app id

Signed-off-by: Ferdinand Thiessen <rpm@fthiessen.de>

README.md

@@ -11,7 +11,7 @@ To install it change into your Nextcloud's apps directory:
 
     cd nextcloud/apps
 
-Then run:
+Then clone this repository into a folder named **notestutorial**¹:
 
     git clone https://github.com/nextcloud/app-tutorial.git notestutorial
 
@@ -19,6 +19,8 @@ Then install the dependencies using:
 
     make composer
 
+¹ It is important that the directory is named exactly like the app ID (see `appinfo/info.xml`).
+
 ## Frontend development
 
 The app tutorial also shows the very basic implementation of an app frontend using [Vue.js](https://vuejs.org/). To build the frontend code after doing changes to its source in `src/` requires to have Node and npm installed.