[service] Document kernel command line arguments

doc/boot_arguments.md

@@ -0,0 +1,28 @@
+# Reading arguments from kernel command line
+
+D-Installer configuration can be altered through the kernel command line. It is possible to load a
+full configuration file or to change some specific values.
+
+## Loading a new configuration file
+
+It is possible to load a new configuration file specifying a URL through the `dinst.config_url`
+option. Here are some examples:
+
+* `dinst.config_url=http://192.168.122.1/my-d-installer.yaml`
+* `dinst.config_url=usb:///d-installer.yaml`
+
+See [URL handling in the
+installer](https://github.com/yast/yast-installation/blob/master/doc/url.md) to find more details
+about the supported URLs.
+
+## Changing configuration values
+
+Instead of loading a full configuration file, you might be interested in adjusting just a few
+configuration values. You must specify the option name in dotted notation. A typical use-case might
+be to use your own SSL certificates:
+
+```
+dinst.web.ssl=true dinst.web.ssl_cert=http://192.168.122.1/mycert.pem dinst.web.ssl_key=http://192.168.122.1/mycert.key
+```
+
+Changing complex options (e.g., collections) is not supported yet.