Document how the installer prompt works.

README.md

@@ -7,3 +7,20 @@ Releases are signed with Simon Quigley's GPG key: 5C7ABEA20F8630459CC8C8B5E27F2C
 Licensing info:
  - Everything is GPL-3 by 2022 Lubuntu Developers <lubuntu-devel@lists.ubuntu.com> unless stated otherwise.
  - img/background.png is licensed CC-BY-4.0, authored by Aaron Rainbolt <arraybolt3@gmail.com>, copyright ownership the same as source
+
+## Architecture
+
+This section serves to explain how lubuntu-installer-prompt's various components work together to provide the Lubuntu ISO boot experience.
+
+1. SDDM loads.
+2. /etc/sddm.conf is read. This file has been generated (or modified?) by Casper to point to a lubuntu-live-environment.desktop X session.
+3. lubuntu-live-environment.desktop executes /bin/start-lubuntu-live-env.
+4. start-lubuntu-live-env starts Openbox and backgrounds it.
+5. start-lubuntu-live-env starts /bin/lubuntu-installer-prompt but does not background it.
+6. The installer prompt appears.
+7. The user clicks "Install Lubuntu" or "Try Lubuntu".
+8. If "Install Lubuntu" is clicked, the installer prompt executes /usr/libexec/lubuntu-installer.sh, which then executes Calamares. The installer prompt then removes the buttons from the screen.
+9. The installer window appears on the user's screen.
+10. If Calamares closes, the installer prompt detects this and exits.
+11. When the installer prompt exits, start-lubuntu-live-env kills Openbox, then runs startlxqt.
+12. If "Try Lubuntu" is clicked, steps 10 and 11 are executed immediately.