Show how to fetch-content dependencies which are not in CMake

[bast]

From #3

An example of how to fetch a content that doesn't support Cmake:

FetchContent_Declare(
the_x_package
GIT_REPOSITORY https://github.com/thepackage.git
)

FetchContent_GetProperties(the_x_package)
if(NOT the_x_package_POPULATED)
FetchContent_Populate(the_x_package)
# Let say it is a header only lib:
add_library(the_x_lib INTERFACE)
target_include_directories(the_x_lib INTERFACE ${the_x_package_SOURCE_DIR})
endif()