docstrings for to_pfm

[gfudenberg]

Thanks for the great tool.

Quick suggestion: it would be nice to have PSEUDO_PFM_COUNT documented in readthedocs, it would have saved some confusion for me about the output of .to_pfm(). It's also not clear what these other constants defined next to PSEUDO_PFM_COUNT are

gimmemotifs/gimmemotifs/motif/__init__.py

Line 60 in 33ade29

PSEUDO_PFM_COUNT = 1000 # JASPAR mean

Thanks again!