Add installation from conda-forge to README

[mantkiew]

Hello @msteinbeck, I created a conda-forge recipe for tinyspline

https://github.com/conda-forge/staged-recipes/pull/26343

in order to make the library available in conda-forge. This recipe is for the C/C++ only.

I also added you as the recipe maintainer. If you would like that, please confirm by posting a comment on the recipe PR. Otherwise, if you don't want to be a maintainer, I'll remove you from the list.

Thanks for the awesome library!

Best, Michal Antkiewicz