More detailed description of FetchContent in README.md

README.md

@@ -261,8 +261,8 @@ CMake controls options with prefixed `-D`, e.g. `-DopenPMD_USE_MPI=OFF`:
 <sup>1</sup> *e.g. changes C++ visibility keywords, breaks MSVC*
 <sup>2</sup> *this includes most pre-/post-condition checks, disabling without specific cause is highly discouraged*
 
-Additionally, the following libraries are shipped internally or, if the
-corresponding `<PACKAGENAME>_ROOT` variable is provided, can be provided externally:
+Additionally, the following libraries are downloaded via [FetchContent](https://cmake.org/cmake/help/latest/module/FetchContent.html)
+during the configuration of the project or, if the corresponding `<PACKAGENAME>_ROOT` variable is provided, can be provided externally:
 * [Catch2](https://github.com/catchorg/Catch2) (2.13.10+)
 * [PyBind11](https://github.com/pybind/pybind11) (2.11.1+)
 * [NLohmann-JSON](https://github.com/nlohmann/json) (3.9.1+)