Docs: explain go templates in custom commands (#428)

docs/Config.md

@@ -31,7 +31,7 @@ gui:
   # Side panel width as a ratio of the screen's width
   sidePanelWidth: 0.333
   # Determines whether we show the bottom line (the one containing keybinding
-	# info and the status of the app).
+  # info and the status of the app).
   showBottomLine: true
   # When true, increases vertical space used by focused side panel,
   # creating an accordion effect
@@ -104,6 +104,11 @@ customCommands:
       serviceNames: []
 ```
 
+You may use the following go templates (such as `{{ .Container.ID }}` above) in your commands:
+- `{{ .DockerCompose }}`: the docker compose command (default: `docker-compose`)
+- [`{{ .Container }}`](https://pkg.go.dev/github.com/jesseduffield/lazydocker@v0.20.0/pkg/commands#Container) and its fields. For example: `{{ .Container.Container.ImageID }}`
+- [`{{ .Service }}`](https://pkg.go.dev/github.com/jesseduffield/lazydocker@v0.20.0/pkg/commands#Service) and its fields. For example: `{{ .Service.Name }}`
+
 ## Replacements
 
 You can add replacements like so: