Add illustration to demonstrate the local reference frame

[jellepoland]

I suggest the image above, taken from: https://doi.org/10.21203/rs.3.rs-3955527/v1

[1-Bart-1]

Nice image! You can ask @ufechner7 to add it to the docs or do it yourself. Just create a branch, implement your changes, and create a pull request.

[ufechner7]

I mean, I can add the pic to https://albatross-kite-transport.github.io/VortexStepMethod.jl/dev/reference_frames/ . But I would also need some text that explains it.

[ufechner7]

If you want to put a pic into the repo, add it to the folder docu/src . If you want to reference it in docu/src/reference_frames.md, add the line:

![ReferenceFrame](reference_frame.png)