Add xxHash example

[RozeFound]

closes #416

[TheLartians]

Hey, thanks for adding the example, this looks like a useful library!
It seems however that it is failing to build on our CI. Could you double-check the code? The error is the following:

  CMake Deprecation Warning at /home/runner/work/CPM.cmake/CPM.cmake/build/xxHash/_deps/xxhash-src/cmake_unofficial/CMakeLists.txt:8 (cmake_minimum_required):
    Compatibility with CMake < 3.5 will be removed from a future version of
    CMake.
  
    Update the VERSION argument <min> value or use a ...<max> suffix to tell
    CMake that the project does not need compatibility with older versions.
  
  
  CMake Error at CMakeLists.txt:22 (target_link_libraries):
    Target "CPMxxHashExample" links to:
  
      xxHash::xxHash
  
    but the target was not found.  Possible reasons include:
  
      * There is a typo in the target name.
      * A find_package call is missing for an IMPORTED target.
      * An ALIAS target is missing.
  
  
  
  CMake Generate step failed.  Build files cannot be regenerated correctly.

Also be sure to run cmake-format. For more info, check out our style guides.

[RozeFound]

I fixed link error, but unfortunately I can't get cmake-format to work for some reason.
Do I also need to fix the warning? It's fixed in one of a post-release commits, so we can simply use this instead of a release tag.

[TheLartians]

I fixed link error, but unfortunately I can't get cmake-format to work for some reason.

Ok, no worries, I've run the code formatters on my machine.

Do I also need to fix the warning? It's fixed in one of a post-release commits, so we can simply use this instead of a release tag.

I like using release tags here, so for now let's live with the warning and update once there is another release.

[TheLartians]

Thanks for adding the example!