Add missing mention of musl to CHANGELOG
- [Add support for `rustup target add all`][pr#1868]
- [Add `rustup show home`][pr#1933]
- [Add NetBSD target to CI][pr#1978]
+- [Add x86_64 musl to CI][pr#1882]
### Thanks
[pr#1873]: https://github.com/rust-lang/rustup.rs/pulls/1873
[pr#1885]: https://github.com/rust-lang/rustup.rs/pulls/1885
+[pr#1882]: https://github.com/rust-lang/rustup.rs/pulls/1882
[pr#1895]: https://github.com/rust-lang/rustup.rs/pulls/1895
[pr#1876]: https://github.com/rust-lang/rustup.rs/pulls/1876
[pr#1901]: https://github.com/rust-lang/rustup.rs/pulls/1901
