Linux also need install clang

[vioulo]

when i build the app at first，i meet some questions.
need to install clang libclang-dev
Adding these to the readme may be more friendly to novices！

[follower]

This is probably due to the recent addition of Text-To-Speech support.

I had an existing issue to note this requirement on the underlying crate (ndarilek/tts-rs#8) but yes, would be good to document in egui README too.

[emilk]

I'll remove the screen_reader from default features. It is quite experimental anyway