Explain you need --features="relm4/libadwaita" to use libadwaita properly

[Dariasteam]

I am absolutely new to rust development so I apologize if this is too obvious to be added to the manual but it took me some time to figure out why the style of my adw widgets were wrong :)

[AaronErhardt]

Currently, the feature flags are documented in the README: https://github.com/Relm4/Relm4#features

I don't think we should copy this documentation to the book because that would probably cause one of the lists to become outdated eventually. However, I think it's a good idea to mention that additional features exist and you might want to use
relm4 = { "0.5.0-beta.4", features = ["libadwaita"] }
for example.

[Dariasteam]

I shouldn't have jumped so fast to the book then. Sorry and thanks!