Add clarifications

[Fil158]

Referring to this parte of the text in the README:

This algorithm is designed to do an NMF factorization on a set of signals ignoring any uniform stretching of the signal on the independent variable axis. For example, for powder diffraction data taken from samples containing multiple chemical phases where the measurements were done at different temperatures and the materials were undergoing thermal expansion.

I think it is not very clear here what the package does and how it can be used to solve the example problem. You are talking about VT-PXRD experiment of samples with different phases which undergo a termal expansion, so a modification of unit cell parameters, each phase independently with respect to the other in the same sample?