Add note about 1.16 branch in README
A Minecraft server implementation written in Rust.
+_Notice_: The codebase is undergoing a major refactor in [this PR](https://github.com/feather-rs/feather/pull/307). The PR will be merged shortly, but the code on the `develop` branch is discontinued.
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### Features
Note that Feather is still early in development. Don't expect anything not listed here to work.
