Update install doc for gentoo
Update install doc for gentoo

prepare/TestWeasyPrint,Kozea/WeasyPrint,marclaporte/WeasyPrint,prepare/TestWeasyPrint,Kozea/WeasyPrint,jasco/WeasyPrint,andrewleech/WeasyPrint,marclaporte/WeasyPrint,andrewleech/WeasyPrint,jasco/WeasyPrint