Add info about new special rooms, such as the library, to wiki

[TommiGustafsson-HMP]

• General information about what kinds of new rooms there are.
• If necessary, detailed information about each room.

[TommiGustafsson-HMP]

Started adding new rooms: https://github.com/hyvanmielenpelit/GnollHack/wiki/New-Rooms

@janne-hmp might want to add technical details for the rooms.