Conceptual Separation of By Regions vs. Whole CDR/FWR analysis

[Robert Amezquita]

http://shazam.readthedocs.io/en/version-0.1.8---mutation-profiling-enhancements/vignettes/Mutation-Vignette/#calculate-mutations-within-subregions-of-the-v-segment

Might be clearer to add a bit of text to separate the usage of IMGT_V vs IMGT_V_BY_REGIONS in calculating FWR/CDR mutation frequencies.

[thanks to Yisi for the pointer]

[Jason Vander Heiden]

Duplicate of #64.