Use FetchContent_MakeAvailable in README (#10)

# Release-tracking repository for nlohmann/json

This repository is based on: [astoeckel/json](https://github.com/astoeckel/json).

The goal is to provide a lightweight repository tracking every releases of

[nlohmann/json](https://github.com/nlohmann/json), compatible with cmake [FetchContent](https://cmake.org/cmake/help/v3.11/module/FetchContent.html).

Goal is to provide a lightweight and autonomous repository tracking every

releases of [nlohmann/json](https://github.com/nlohmann/json).

It is meant to be used with CMake

[FetchContent](https://cmake.org/cmake/help/v3.11/module/FetchContent.html).

You can always replace the URL by the official repository:

<https://github.com/nlohmann/json>.

The only differences are:

* The download size: ~500KB vs ~150MB (300× difference)

* Some options are not available. See [the unsupported options section](#Unsupported-options).

You can depends on it using:

## Example

**Example**:

~~~cmake

include(FetchContent)

# Optional: set this to ON if your target publicly links to nlohmann_json and needs to install() 

# set(JSON_Install ON)

FetchContent_Declare(json

  GIT_REPOSITORY https://github.com/ArthurSonzogni/nlohmann_json_cmake_fetchcontent

  GIT_PROGRESS TRUE

  GIT_SHALLOW TRUE

  GIT_TAG v3.10.4)

FetchContent_GetProperties(json)

if(NOT json_POPULATED)

  FetchContent_Populate(json)

  add_subdirectory(${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)

endif()

FetchContent_MakeAvailable(json)

target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

~~~

You can replace always replace the URL by the official repository:

https://github.com/nlohmann/json

The only difference is the download size. It would be several orders of magnitude

larger.

## Unsupported options

The following options are currently not supported. This is done on purpose

because they do not really make sense for a mirror repository, or they have not

been thoroughly tested. Consider using the official repository if you need these

options.

* `JSON_CI`

* `JSON_BuildTests`

## Updates

This repository is fully autonomous. It updates itself every week using github

actions.

See:

- [#2073](https://github.com/nlohmann/json/issues/2073),

- [#732](https://github.com/nlohmann/json/issues/732),

- [#620](https://github.com/nlohmann/json/issues/620),

- [#556](https://github.com/nlohmann/json/issues/556),

- [#482](https://github.com/nlohmann/json/issues/482),

- [#96](https://github.com/nlohmann/json/issues/96)

### Thanks

This repository is based on: [astoeckel/json](https://github.com/astoeckel/json).

### Addressed `nlohmann/json` issues:

* [#2073](https://github.com/nlohmann/json/issues/2073),

* [#732](https://github.com/nlohmann/json/issues/732),

* [#620](https://github.com/nlohmann/json/issues/620),

* [#556](https://github.com/nlohmann/json/issues/556),

* [#482](https://github.com/nlohmann/json/issues/482),

* [#96](https://github.com/nlohmann/json/issues/96)