doc: add doc about CPM

Author: ToruNiina

README.md

@@ -109,14 +109,27 @@ include(FetchContent)
 FetchContent_Declare(
   toml11
   GIT_REPOSITORY https://github.com/ToruNiina/toml11.git
-  GIT_TAG        v4.0.3
+  GIT_TAG        v4.1.0
 )
 FetchContent_MakeAvailable(toml11)
 
 add_executable(main main.cpp)
 target_link_libraries(main PRIVATE toml11::toml11)
 ```
 
+### CMake Package Manager (CPM)
+
+After [adding cpm to your project](https://github.com/cpm-cmake/CPM.cmake?tab=readme-ov-file#adding-cpm), you can use toml11 by doing:
+
+```cmake
+include(cmake/CPM.cmake)
+
+CPMAddPackage("gh:ToruNiina/toml11@4.1.0")
+
+add_executable(main main.cpp)
+target_link_libraries(main PUBLIC toml11::toml11)
+```
+
 ### Install Using CMake
 
 You can install toml11 using CMake with the following steps:

README_ja.md

@@ -114,6 +114,19 @@ add_executable(main main.cpp)
 target_link_libraries(main PRIVATE toml11::toml11)
 ```
 
+### CMake Package Manager (CPM)
+
+[CMake package manager](https://github.com/cpm-cmake/CPM.cmake)を導入すると、以下のようにして使用することができます。
+
+```cmake
+include(cmake/CPM.cmake)
+
+CPMAddPackage("gh:ToruNiina/toml11@4.1.0")
+
+add_executable(main main.cpp)
+target_link_libraries(main PUBLIC toml11::toml11)
+```
+
 ### Install using CMake
 
 以下の手順で、CMakeを使ってインストールすることができます。