From Elite Wiki

shipyard-overrides.plist is added with oolite 1.72.

It loads after all the shipyard data are read in. It contains the same keys as the normal shipyard.plist. Whenever it finds a match with a key that also exists in shipyard.plist of any oxp, it overwrites the entries defined in shipyard-overrides.plist but leaves the other as they were. e.g.

"adder-player" = {
 chance = 0.35;
 techlevel = 6;

Will change the techlevel and the chance of appearance of the ship adder-player. All other data remain original

See OXP_howto for a overview about the other plists.