Define minimum supported rust version

Detected automatically via `cargo msrv`

The usage of the thread_local dependency appears to be what constrains us here.

Author: Techcable

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 * Call of deprecated `err.description()` replaced with `err.to_string()`.
 * Avoided all catchable panics in async drain thread.
+* Define minimum supported rust version.
 
 ## 2.7.0 - 2021-07-29
 

Cargo.toml

@@ -10,6 +10,7 @@ documentation = "https://docs.rs/slog-async"
 homepage = "https://github.com/slog-rs/slog"
 repository = "https://github.com/slog-rs/async"
 readme = "README.md"
+rust-version = "1.59.0"
 
 [features]
 nested-values = ["slog/nested-values"]