The guide, and Twilight as a whole, assume familiarity with Rust, Rust's asynchronous features, and bots in general. If you're new to Rust and/or new to bots, consider checking out serenity, which is a beginner-friendly, batteries-included approach to the Discord API.
If you have a question, then the issues or the server are both good fits for it. If you find a bug, then the issues section is the best place.
The API documentation is also available.
Twilight currently supports the latest stable Rust version.
Although Twilight aims to design APIs right the first time, that obviously won't happen. A lot of effort is spent designing clear and correct interfaces.
While Twilight takes care to avoid the need for breaking changes, it will be fearless when it needs to do so: they won't be avoided for the sake of avoiding a change. Breaking changes won't be piled up over time to make a single big release: major versions will be often and painless.