README.rst: Add VIGRA as an optional dependency.
README.rst: Add VIGRA as an optional dependency.

nanshe-org/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter,jakirkham/rank_filter,DudLab/rank_filter,jakirkham/rank_filter,DudLab/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter