update installation instructions to clarify that repository needs to be cloned
update installation instructions to clarify that repository needs to be cloned

jsannemo/problemtools,godmar/problemtools,Kattis/problemtools,ghamerly/problemtools,godmar/problemtools,ghamerly/problemtools,ghamerly/problemtools,jsannemo/problemtools,ghamerly/problemtools,godmar/problemtools,Kattis/problemtools,godmar/problemtools,Kattis/problemtools,jsannemo/problemtools,Kattis/problemtools,jsannemo/problemtools