Update README.md

README.md

@@ -147,6 +147,7 @@ coeffs = sht(signal)
 Schaeffer, N.;
 Efficient spherical harmonic transforms aimed at pseudospectral numerical simulations;
 G3: Geochemistry, Geophysics, Geosystems.
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 <a id="1">[2]</a> 
 Wang, B., Wang, L., Xie, Z.;
 Accurate calculation of spherical and vector spherical harmonic expansions via spectral element grids;