Merge pull request #1354 from cgwalters/doc-bootloader

docs: Add bootloaders.md

Author: cgwalters

docs/src/SUMMARY.md

@@ -51,6 +51,7 @@
 - [Filesystem](filesystem.md)
 - [Filesystem: sysroot](filesystem-sysroot.md)
 - [Container storage](filesystem-storage.md)
+- [Bootloader](bootloader.md)
 
 # Experimental features
 

docs/src/bootloaders.md

@@ -0,0 +1,9 @@
+# Bootloaders in `bootc`
+
+`bootc` uses [bootupd](https://github.com/coreos/bootupd/) by default to manage bootloader installation and configuration. `bootupd` is an external project that abstracts over bootloader installs and upgrades, providing a consistent interface for different bootloader types (e.g., GRUB, systemd-boot).
+
+When you run `bootc install`, it invokes `bootupctl backend install` to install the bootloader to the target disk or filesystem. The specific bootloader configuration is determined by the container image and the target system's hardware.
+
+Currently, `bootc` only runs `bootupd` during the installation process. It does **not** automatically run `bootupctl update` to update the bootloader after installation. This means that bootloader updates must be handled separately, typically by the user or an automated system update process.
+
+For s390x, bootc uses `zipl` instead of `bootupd`.