[18USA] Add tile information to P16 - Regional Headquarters

[ChrisTheCat]

Regional Headquarters may be used to upgrade a green or brown non-metropolis city after phase 5 begins. May be placed on any existing normal city. Three of the track segments are optional and can be placed pointing toward water or other off map areas.

There are no cities on the map that require three optional track segments. Also, it would help if this mentioned that there is a Regional Headquarters tile, thus:

Regional Headquarters may be used to upgrade a green or brown non-metropolis city after phase 5 begins. Tile #X23 RHQ may be placed on any existing normal city. Two of the track segments are optional and can be placed pointing toward water or other off map areas.