Correct README slightly
Druid is a new Rust-native UI toolkit, still in early stages. Its main
goal is performance, also aiming for small binary size and compile time,
fast startup, and very easy build configuration (just `cargo run`).
-It currently supports Windows and macOS, with GNU/Linux support planned.
+It currently supports Windows, macOS, and GNU/Linux.
Raph gave a talk at the July 2018 SF Rust Meetup ([video][jul-2018-video],
[slides][jul-2018-slides]) about the design. Traditional object-oriented
