README: add installer customizations

Add an example for installer customizations to the README.  Include the
same example in both toml and json format for better readability.
Unlike other examples, the toml format is shown first since it's easier
to work with.

Add a note for users explaining that the ostreecontainer command is
automatically added to the kickstart file when building an ISO.  Mention
that no other command is added so the user should include any other
options they need.

Link to the docs for up-to-date information on interactions with other
commands.

README.md

@@ -332,6 +332,38 @@ Example:
 
 ```
 
+### Anaconda ISO (installer) options (`installer`, mapping)
+
+Users can include kickstart file content that will be added to an ISO build to configure the installation process.
+Since multi-line strings are difficult to write and read in json, it's easier to use the toml format when adding kickstart contents:
+
+```toml
+[customizations.installer.kickstart]
+contents = """
+text --non-interactive
+zerombr
+clearpart --all --initlabel --disklabel=gpt
+autopart --noswap --type=lvm
+network --bootproto=dhcp --device=link --activate --onboot=on
+"""
+```
+
+The equivalent in json would be:
+```json
+{
+  "customizations": {
+    "installer": {
+      "kickstart": {
+        "contents": "text --non-interactive\nzerombr\nclearpart --all --initlabel --disklabel=gpt\nautopart --noswap --type=lvm\nnetwork --bootproto=dhcp --device=link --activate --onboot=on"
+      }
+    }
+  }
+}
+```
+
+Note that bootc-image-builder will automatically add the command that installs the container image (`ostreecontainer ...`), so this line or any line that conflicts with it should not be included. See the relevant [Kickstart documentation](https://pykickstart.readthedocs.io/en/latest/kickstart-docs.html#ostreecontainer) for more information.
+No other kickstart commands are added by bootc-image-builder in this case, so it is the responsibility of the user to provide all other commands (for example, for partitioning, network, language, etc).
+
 ## Building
 
 To build the container locally you can run