Add crate level doc for feature serde

SpriteOvO · SpriteOvO · commit 41fd38652340 · 2024-10-28T21:48:15.000+08:00
diff --git a/spdlog/src/lib.rs b/spdlog/src/lib.rs
@@ -207,6 +207,8 @@
 //!  - `runtime-pattern` enables the ability to build patterns with runtime
 //!    template string. See [`RuntimePattern`] for more details.
 //!
+//!  - `serde` enables [`formatter::JsonFormatter`].
+//!
 //! # Supported Rust versions
 //!
 //! <!--
