Add games/openrct2.

OpenRCT2 is an open source reimplementation of the game engine used by
RollerCoaster Tycoon 2 (RCT2).

The gameplay revolves around building and maintaining an amusement park
containing attractions, shops and facilities. The player must try to make
a profit and maintain a good park reputation whilst keeping the guests happy.

OpenRCT2 allows for both scenario and sandbox play. Scenarios require the
player to complete a certain objective in a set time limit whilst sandbox
allows the player to build a more flexible park with optionally no restrictions
or finance.

OpenRCT2 requires a copy of RollerCoaster Tycoon 2 to play.

(nia)