Missing explaination for changing the contrast coding in documentation

[maxvanmigem]

There is no explanation or reference to how you can change the coding contrasts for categorical predictiors in the unfold model in the documentation.

[behinger]

Thanks for reporting!

@all-contributors please add @maxvanmigem for bug

[allcontributors]

@behinger

I've put up a pull request to add @maxvanmigem! 🎉