add minimum minetest version key for contentdb

mod.conf

@@ -2,3 +2,4 @@ name = pipeworks
 description = This mod uses mesh nodes and nodeboxes to supply a complete set of 3D pipes and tubes, along with devices that work with them.
 depends = default, basic_materials, screwdriver
 optional_depends = mesecons, mesecons_mvps, digilines, signs_lib
+min_minetest_version = 5.2.0