add description how to test extractor in with frontend

docs/04_Run_changes_in_App.md

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+# Run the changes in the App
+
+You should develop and test your changes with the JUnit environment that is
+provided by the NewPipe Extractor and IDEA. If you want to try it then with
+the actual fronted you need to follow these changes.
+
+### Setup Android Studio
+
+First of all you want to setup a working Android Studio environment. For this please
+download Studio from [developer.android.com](https://developer.android.com/studio/),
+and follow the [instructions](https://developer.android.com/studio/install) about how to set it up.
+
+### Get the NewPipe code and run it.
+
+In order to get it you must simply clone or download from the current `dev` branch
+[github.com/TeamNewPipe/NewPipe.git](https://github.com/TeamNewPipe/NewPipe/archive/dev.zip).
+You can then build and run it following [these instructions](https://developer.android.com/studio/run/).
+Please also make sure you are comfortable with [adb](https://en.droidwiki.org/wiki/Android_Debug_Bridge) since
+you might experience some trouble running your compiled app on a real device, especially under linux where you
+sometimes have to adjust the udev rules in order to
+[make your device be accessible](http://www.janosgyerik.com/adding-udev-rules-for-usb-debugging-android-devices/).
+
+### Run your changes on the Extractor
+
+In order to use the extractor in our app we use [jitpack](https://jitpack.io). This is a build service that can build
+marven *.jar packages for android and java based on a github or gitlab repositories. 
+
+To the extractor through jitpack, you need to push them to your online repository of
+your copy that you host either on [github](https://github.com) or [gitlab](https://gitlab.com). It's important to host
+it on one of both. Now copy your repository url in Http format, go to [jitpack](https://jitpack.io/), and past it there
+From here you can grap the latest commit via `GET IT` button.
+I recomend not to use SNAPSHOT, since I am not sure when snapshot is build. An "implementation" string will be generated
+for you. Copy this string and replace the `implementation 'com.github.TeamNewPipe:NewPipeExtractor:<commit>'` line in
+the file [/app/build.gradle](https://github.com/TeamNewPipe/NewPipe/blob/dev/app/build.gradle#L58) with it.
+
+<video width="600" controls>
+  <source src="/media/how_to_jitpack.mp4" type="video/mp4">
+Your browser does not support the video tag.
+</video>
+
+If everything synced well then you should only see a screen with OK signs. Now you can go on an compile and run NewPipe
+with the new extractor.
+![image_sync_ok](/img/sync_ok.png)
+
+### Trouble shoot
+
+If something went wrong on jitpack site, you can check their build log, by selecting the commit you tried to build and
+click on that little paper symbol next to the `GET IT` button. If it is red it already shows that the build failed.
+![jitpack failed to build](/img/jitpack_fail.png)