Updated comment about mdc

include/spdlog/mdc.h

@@ -1,10 +1,20 @@
+// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+
 #pragma once
 
 #include <map>
 #include <string>
 
 #include <spdlog/common.h>
 
+// MDC is a simple map of key->string values stored in thread local storage whose content will be printed by the loggers.
+// Note: Not supported in async mode (thread local storage - so the async thread pool have different copy).
+//
+// Usage example:
+// spdlog::mdc::put("mdc_key_1", "mdc_value_1");
+// spdlog::info("Hello, {}", "World!");  // => [2024-04-26 02:08:05.040] [info] [mdc_key_1:mdc_value_1] Hello, World!
+
 namespace spdlog {
 class SPDLOG_API mdc {
 public: