Change to Third Party part policy

[Eugen]

This policy and the list of companies that have authorized or refused inclusion will be found in the LDraw.org Library Policies document:
https://www.ldraw.org/pt-policies.html

I will be working to contact all the affected companies in the coming weeks.

As there are already some changes in the Third Party list, would be good add a note:

The following companies are not authorized for the Parts Library
(some of them are providing own LDraw files under other terms)