docs: document the mechanism for after installation setup

doc/after_installation_setup.md

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+# Further configuration
+
+TL;DR Agama will support running post-installation scripts after the first boot. No specific support
+for a CMS[^cms], Combustion/Ignition, etc.
+
+It is rather typical that users want to perform some additional configuration steps once the
+installation is finished. To cover those use cases, AutoYaST offers support for running
+[user-defined
+scripts](https://documentation.suse.com/sles/15-SP5/html/SLES-all/cha-configuration-installation-options.html#createprofile-scripts)
+and the [YaST2 Configuration Management
+module](https://github.com/yast/yast-configuration-management/).
+
+But what are the plans for Agama? This document describes Agama's take on this topic, including the
+rationale behind the decision.
+
+[^cms]: Configuration Management System
+
+## Using user-defined scripts
+
+Enabling users to run their own scripts is the minimal Agama could do. With that support, a user can
+decide what to do with them. Actually, SUMA and many customers already follow this approach with
+AutoYaST.
+
+Furthermore, it looks like a must if we want to offer a good level of backward compatibility with
+AutoYaST. However, whether we should unify the `post-installation` and `init-scripts` into a single
+type of scripts is debatable.
+
+## Setting up a CMS
+
+The [YaST2 Configuration Management module](https://github.com/yast/yast-configuration-management/)
+can set up the system to connect to a CMS. It even supports [Salt parametrizable
+formulas](https://imobachgs.github.io/yast/2017/03/01/yast2-cm-gets-support-for-salt-parametrizable-formulas.html)
+since 2017. However, we have the impression that no one uses them in the context of AutoYaST.
+
+As this set up can be achieved by using a user-defined script, we will not implement specific
+support for any CMS (at least by now).
+
+## Using Combustion/Ignition
+
+[openSUSE MicroOS](https://microos.opensuse.org/) images rely on Combustion (or Ignition) to perform
+additional configuration steps, so we considered to use it in Agama too. However, you cannot use the
+root file system to provide the Combustion scripts (and additional assets). You need to provide an
+additional source (a URL, a USB stick, or specific
+[QEMU](https://www.qemu.org/docs/master/specs/fw_cfg.html) and
+[VirtualBox](https://docs.vmware.com/en/VMware-Tools/12.4.0/com.vmware.vsphere.vmwaretools.doc/GUID-D026777B-606D-4442-957A-B953C2049659.html)
+mechanisms).
+
+For that reason, Agama will not rely on Combustion by now.
+
+## References
+
+- [PED-10827](https://jira.suse.com/browse/PED-10827)