add second paper

README.md

@@ -1,5 +1,7 @@
 A Python package aiming to provide an easy and efficient interface to various implementations of the [Hexagonal Fast Fourier Transform](https://en.wikipedia.org/wiki/Hexagonal_fast_Fourier_transform). The purpose of the software is primarily educational with a goal of eventually reaching production quality.
 
-For now the `fft()` and `ifft()` methods implement the HFFT algorithm due to Mersereau in
+For now the `fft()` and `ifft()` methods implement HFFT described in the following papers:
 
 > R. M. Mersereau, "The processing of hexagonally sampled two-dimensional signals," in Proceedings of the IEEE, vol. 67, no. 6, pp. 930-949, June 1979, doi: 10.1109/PROC.1979.11356
+
+> Ehrhardt, J. C. (1993). Hexagonal fast Fourier transform with rectangular output. In IEEE Transactions on Signal Processing (Vol. 41, Issue 3, pp. 1469–1472). Institute of Electrical and Electronics Engineers (IEEE). https://doi.org/10.1109/78.205759