update runal docs

Author: Xavier Godart

content/runal.md

@@ -110,6 +110,16 @@ The js file will be **automatically reloaded when modified**, no need to restart
 
 To **exit**, just type `ctrl+c`.
 
+#### Standalone executable
+
+You can create a standalone executable from the JavaScript file specified with **-f** using **-o [FILE]**:
+```sh
+./runal -f sketch.js -o sketch
+
+# Run the standalone executable
+./sketch
+```
+
 ### Go package
 
 Because **Runal** is written in Go, you can also use it as a Go package.