update ChangeLog

ChangeLog

 Changes since NIFTy 5:
 
+FFT convention adjusted
+=======================
+
+When going to harmonic space, NIFTy's FFT operator now uses a minus sign in the
+exponent (and, consequently, a plus sign on the adjoint transform). This
+convention is consistent with almost all other numerical FFT libraries.
+
 MPI parallelisation over samples in MetricGaussianKL
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