Add "include(FetchContent)"-line to README.md

[niekbouman]

Dear Rob,

Thank you for contributing this repo. For me it is extremely useful!
Just one little note, under the "Using in your project" part of the README.md, you could add

   include(FetchContent)

which could help users unfamiliar with CMake's FetchContent machinery (like myself, before coming across your repo..!)

kind regards,
Niek

[robinlinden]

Sure thing! It would be good if the example were self-contained, including the minimum_version, I suppose. :)

I'm happy to hear you think the project is useful!