pdm install instructions unclear

[marianne013]

• I got the install script via wget https://raw.githubusercontent.com/ic-hep/pdm/master/src/pdm/installer/install_pdm.py
  If this is the correct way, then wget should go among the required packages and the above command should be included in the instructions. If not, the please provide alternative instructions.
• pdm.conf should contain examples, e.g. for #service_url = (in fact an example config file in the documentation would probably be really useful -- I can add one, once I worked it out).
• Not clear if I need to un-comment e.g. #conf_dir = /etc/pdm if I just want to stick with the default config
• All variables that are necessary for registration should be marked as such.
• Every variable should have a short note what it's for, please don't make me play "hunt the variable".