Update install-gnu.md: Added AM AppImage management guide

docs/guides/install-gnu.md

@@ -18,7 +18,107 @@ installing RetroArch on systems running the GNU/Linux kernel.
 
 ---
 
-## Flatpak (suitable for most Linux distributions)
+## AppImage (suitable for most GNU/Linux distributions)
+
+### AM
+
+[AM](https://github.com/ivan-hc/AM) is a powerful command-line package manager for managing AppImage software. It allows users to install, update, and remove AppImages either system-wide or locally without manual setup.
+
+1. Installation
+
+There are no system packages available for AM in common repositories, so it must be installed manually.
+
+Follow the official installation instructions provided at: [https://github.com/ivan-hc/AM](https://github.com/ivan-hc/AM)
+
+During setup, you’ll be prompted to choose one of two installation modes:
+
+* Option 1: AM – for system-wide AppImage management (requires root privileges).
+* Option 2: appman – for local, user-level AppImage management (no root access required).
+
+Choose the option that best suits your environment and permissions.
+
+2. Searching for AppImages
+
+Once installation is complete, verify that AM or AppMan is working by performing a search.
+Use the appropriate command depending on your installation:
+
+```
+# For system-wide installation
+am -q retroarch
+
+# For user-level installation
+appman -q retroarch
+
+```
+
+This query searches for available AppImages matching “retroarch” and displays a list of results.
+Example output:
+
+```
+$ appman -q retroarch
+
+  SEARCH RESULTS FOR "RETROARCH":
+
+  ◆ retroarch-nightly : RetroArch is a free and open-source, cross-platform frontend for emulators, game engines, video games, media players and other applications.
+  ◆ retroarch-qt-nightly : RetroArch is a free and open-source, cross-platform frontend for emulators, game engines, video games, media players and other applications.
+  ◆ retroarch-qt : RetroArch is a free and open-source, cross-platform frontend for emulators, game engines, video games, media players and other applications.
+  ◆ retroarch : RetroArch is a free and open-source, cross-platform frontend for emulators, game engines, video games, media players and other applications.
+
+```
+
+3. Installing an AppImage
+
+To install your chosen AppImage, use the `-i` flag followed by the specific package name. For example, you can install either the stable release (for everyday use) or the Nightly build (ideal for testing):
+
+```bash
+# For system-wide installation (Stable)
+am -i retroarch
+
+# For system-wide installation (Nightly)
+am -i retroarch-nightly
+
+# For user-level installation (Stable)
+appman -i retroarch
+
+# For user-level installation (Nightly)
+appman -i retroarch-nightly
+```
+
+4. Updates
+
+To update RetroArch, use the `-u` (or `--update`) flag. Running the update command without specifying a package name will update all installed AppImages managed by the tool:
+
+```bash
+# Update all system-wide AppImages
+am -u
+
+# Update all user-level AppImages
+appman -u
+
+```
+
+
+Alternatively, you can target a specific package to update only that application. This is especially useful if you have multiple Continuous Integration (CI) AppImages of other software installed that you rarely use. Since the [official bug report template](https://github.com/libretro/RetroArch/blob/master/.github/ISSUE_TEMPLATE/01-bug-report.yml) encourages using the latest AppImage, updating just `retroarch-nightly` to test your issue on the latest build before reporting is recommended:
+
+
+```bash
+# For system-wide installation
+am -u retroarch-nightly
+
+# For user-level installation
+appman -u retroarch-nightly
+
+```
+
+> **Note:** Make sure to use the exact package name that you originally installed (e.g., `retroarch`, `retroarch-nightly`, `retroarch-qt`, or `retroarch-qt-nightly`).
+
+5. Notes and Recommendations
+
+* Nightly builds are recommended when submitting bug reports or testing new features.
+* Stable releases are better suited for everyday use.
+* Use the same tool (am or appman) for all lifecycle tasks—installation, updates, and removal.
+
+## Flatpak (suitable for most GNU/Linux distributions)
 
 Flatpak is a distribution-agnostic packaging format with broad support
 throughout the Linux ecosystem. An official [RetroArch
@@ -34,7 +134,7 @@ installed in just three easy steps:
     flatpak --installations
     ```
 
-1. Confirm that the Flathub repository is configured as a [flatpak
+2. Confirm that the Flathub repository is configured as a [flatpak
    remote][flatpak-remote], so that packages from it may be installed. You can
    examine the flatpak remotes currently enabled on your system with this
    terminal command (shown with default output):
@@ -53,7 +153,7 @@ installed in just three easy steps:
     sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
     ```
 
-1. Finally, install the RetroArch Flatpak. You have the option of making it
+3. Finally, install the RetroArch Flatpak. You have the option of making it
    available to only the current user, with this command:
 
     ``` shell