Documentation? In -my- Cataclysm? doc/MAPGEN.md added. Minor syntax change to jsonmapgen, as well as optional "lua" script

Documentation (!!!!) for mapgen added as doc/MAPGEN.md

mapgen_function_json: Minor syntax switching;
'"set": [ { "set": ...' ==> '"set": [ { "point": ...'
'"spawn_items": [ { "id": "itype", ...' ==> '"add": [ { "item": "itype", ...'

mapgen_function_json: Optionally run lua after json stuff ends, and before rotation, with:
"lua": "string" or
"lua": [ "array", "o", "lines" ]