clean up doc dir (AtsushiSakai#567)

Author: AtsushiSakai

README.md

@@ -117,7 +117,7 @@ For development:
 
 - mypy (for type check)
 
-- Sphinx (for document generation)
+- sphinx (for document generation)
 
 - pycodestyle (for code style check)
 

requirements.txt

@@ -3,4 +3,7 @@ scipy == 1.7.2
 pandas == 1.3.4
 matplotlib == 3.4.3
 cvxpy == 1.1.17
+pytest == 6.2.5 # For unit test
+pytest-xdist == 2.4.0 # For unit test
+sphinx == 4.3.0 # For sphinx documentation
 IPython == 7.29.0 # For sphinx documentation