Docs: add info about how to install and update using Obtainium

docs/get-from-github.md

@@ -0,0 +1,14 @@
+# Download and update SeriesGuide via GitHub
+
+The Android Package (APK) for each stable release is available attached to 
+[GitHub releases](https://github.com/UweTrottmann/SeriesGuide/releases) on this repository.
+
+However, it is recommended to use a manager app to ensure the latest updates are installed 
+automatically or you are notified about new updates.
+
+For example, install [Obtainium](https://obtainium.imranr.dev/) and add the SeriesGuide app using
+the following URL:
+
+```text
+https://github.com/UweTrottmann/SeriesGuide
+```