doc: add readme to tools/expand

tools/expand/README.md

@@ -0,0 +1,14 @@
+# expand
+
+`expand` is a tool designed to process `#include "..."` directives within a file, generating a single `single_include/toml.hpp` file.
+
+`expand` is optimized specifically for the structure of `toml11`.
+It assumes that the argument provided is `include/toml.hpp` and that there are files separated into `fwd/` and `impl/` directories.
+Therefore, it is not a general-purpose tool.
+
+## Usage
+
+```console
+$ g++-13 -std=c++20 -O2 main.cpp -o expand
+$ ./expand ../../include/toml.hpp > ../../single_include/toml.hpp
+```