Added instructions to enable the psql intarray extensions
Added instructions to enable the psql intarray extensions

The 'intarray' extension is needed for osm2pgsql to support the osm chage
files (aka. "diffs"). This patch documents how to enable it.

ccnz/eqnz-ocitysmap,maposmatic/ocitysmap,dittaeva/ocitysmap,dittaeva/ocitysmap,wonderchook/OcitySMaps,maposmatic/ocitysmap,maposmatic/ocitysmap