improve documentation

README.md

doc/_build/doctrees/_autosummary/jass_preprocessing.doctree

doc/_build/doctrees/_autosummary/modules.doctree

doc/_build/doctrees/environment.pickle

doc/_build/doctrees/index.doctree

doc/_build/html/.buildinfo

doc/_build/html/_autosummary/jass_preprocessing.html

doc/_build/html/_autosummary/modules.html

doc/_build/html/_modules/index.html

...build/html/_modules/jass_preprocessing/compute_score.html

doc/_build/html/_modules/jass_preprocessing/map_gwas.html

...build/html/_modules/jass_preprocessing/map_reference.html

doc/_build/html/_modules/jass_preprocessing/save_output.html

doc/_build/html/_sources/index.rst.txt

doc/_build/html/_static/basic.css

doc/_build/html/_static/doctools.js

doc/_build/html/_static/documentation_options.js

doc/_build/html/_static/jquery.js

doc/_build/html/_static/pygments.css

doc/_build/html/_static/searchtools.js