[documentation] Improve documentation for kiss_fastfir() overlap-save

[CLafitte]

This PR addresses a TO DO item in the README.md, specifically the need for more explanatory documentation of the FFT in kiss_fastfir.c. This change adds a friendly comment block above kiss_fastfir(), including an ASCII diagram, outlining the steps in the transformation.

Nothing in the actual code changes—this is purely documentation. The goal is just to make it easier for new contributors to understand how the fast convolution works.