README: Clarify 'external_libraries' is under '[options]'
At least I made the mistake of defining 'external_libraries' above
'[options]' and I have a feeling that others will make the same mistake.
Hence this patch.
@@ -35,6 +35,7 @@ And that's all! Neither `GtkWidget` nor `GtkWindow` (alongside with their functi
You also need to add any needed external libraries in the "external_libraries" parameter. Example:
```toml
+[options]
external_libraries = [
"GLib",
"GObject",
