Update readme with custom flags example

README.md

@@ -284,6 +284,39 @@ void user_defined_example()
 }
 
 ```
+
+---
+#### User defined flags in the log pattern
+```c++ 
+// Log patterns can contain custom flags.
+// the following example will add new flag '%*' - which will be bound to a <my_formatter_flag> instance.
+#include "spdlog/pattern_formatter.h"
+class my_formatter_flag : public spdlog::custom_flag_formatter
+{
+public:
+    void format(const spdlog::details::log_msg &, const std::tm &, spdlog::memory_buf_t &dest) override
+    {
+        std::string some_txt = "custom-flag";
+        dest.append(some_txt.data(), some_txt.data() + some_txt.size());
+    }
+
+    std::unique_ptr<custom_flag_formatter> clone() const override
+    {
+        return spdlog::details::make_unique<my_formatter_flag>();
+    }
+};
+
+void custom_flags_example()
+{
+
+    using spdlog::details::make_unique; // for pre c++14
+    auto formatter = make_unique<spdlog::pattern_formatter>();
+    formatter->add_flag<my_formatter_flag>('*').set_pattern("[%n] [%*] [%^%l%$] %v");
+    spdlog::set_formatter(std::move(formatter));
+}
+
+```
+
 ---
 #### Custom error handler
 ```c++
@@ -295,6 +328,8 @@ void err_handler_example()
 }
 
 ```
+
+
 ---
 #### syslog 
 ```c++