Bump version to 0.3.1, update README
Bump version to 0.3.1, update README

ovaskevich/PyLaTeX,ovaskevich/PyLaTeX,sebastianhaas/PyLaTeX,sebastianhaas/PyLaTeX,bjodah/PyLaTeX,votti/PyLaTeX,jendas1/PyLaTeX,bjodah/PyLaTeX,JelteF/PyLaTeX,jendas1/PyLaTeX,JelteF/PyLaTeX,votti/PyLaTeX