README: add explanation on how to use pkg-config.

Author: ericonr

README.md

@@ -27,6 +27,7 @@
 - [Integration](#integration)
   - [CMake](#cmake)
   - [Package Managers](#package-managers)
+  - [Pkg-config](#pkg-config)
 - [Examples](#examples)
   - [JSON as first-class data type](#json-as-first-class-data-type)
   - [Serialization / Deserialization](#serialization--deserialization)
@@ -230,6 +231,20 @@ Please file issues [here](https://github.com/build2-packaging/nlohmann-json) if
 
 If you are using [`wsjcpp`](https://wsjcpp.org), you can use the command `wsjcpp install "https://github.com/nlohmann/json:develop"` to get the latest version. Note you can change the branch ":develop" to an existing tag or another branch.
 
+### Pkg-config
+
+If you are using bare Makefiles, you can use `pkg-config` to generate the include flags that point to where the library is installed:
+
+```sh
+pkg-config nlohmann_json --cflags
+```
+
+Users of the Meson build system will also be able to use a system wide library, which will be found by `pkg-config`:
+
+```meson
+json = dependency('nlohmann_json', required: true)
+```
+
 ## Examples
 
 Beside the examples below, you may want to check the [documentation](https://nlohmann.github.io/json/) where each function contains a separate code example (e.g., check out [`emplace()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a5338e282d1d02bed389d852dd670d98d.html#a5338e282d1d02bed389d852dd670d98d)). All [example files](https://github.com/nlohmann/json/tree/develop/doc/examples) can be compiled and executed on their own (e.g., file [emplace.cpp](https://github.com/nlohmann/json/blob/develop/doc/examples/emplace.cpp)).