Update README.md

Update README.md

Adding link for v0.51

Version 0.51. Fixing bug in reported motifs.

There was a bug present in how ChExMix reports the motif instances in
events files. This bug only affected reporting - no issues expected in
binding event call reproducibility.

Update README.md

Adding version 0.5

Editing PotentialRegionFilter

Adding extra filter step at the very start to remove blacklist regions
from chromosomes before potential regions have been examined. It should
be cleaner to handle cases where a large potential region overlaps an
excluded region this way.