documentation for creating pool accounts on WNs

[paolini78]

there is the need to document how to create pool accounts on WNs.
for example there is this old documentation that still refers to the good old YAIM:
https://twiki.cern.ch/twiki/bin/view/LCG/YaimGuide301#users_conf

In case of other middlware products, the creation of pool accounts could be automatic when enabling a VO, and the documentation could already exist, but it is worth to check if it is the case and to add a link to it

[sebastian-luna-valero]

In case it helps:
https://www.nordugrid.org/arc/arc7/admins/reference.html#mapping-block