docs: Add docs for customizing installers

Add a minimal doc that shows how to customize an installer ISO and what
options there are.

Signed-off-by: Fredrik Lönnegren <[email protected]>

Author: frelon

docs/customizing-installers.md

@@ -0,0 +1,17 @@
+# Customizing installers
+
+Elemental installer images can be customized using the `elemental3 customize` command.
+
+The following command takes an input (./build/installer.iso) and generates a
+new installer2.iso in the output directory including a new config-script, an
+overlay and some extra kernel cmdline parameters:
+
+```sh
+./build/elemental3 customize \
+    --input ./build/installer.iso
+    --output ./build
+    --name installer2
+    --cmdline="console=ttyS0"
+    --config ./examples/elemental/install/config.sh
+    --overlay tar:./build/overlays.tar.gz
+```