NEW: games/dsda-doom 0.25.6

Hi,

I made this port for DSDA-Doom, a relatively new DOOM source port with
focus on speedrunning.

Is it OK?
(I have no experience in writing ports; this is my first one :)

Article on the DOOM Wiki: https://doomwiki.org/wiki/DSDA-Doom
Post on the DoomWorld forums: https://www.doomworld.com/vb/thread/118074
Repository on Github: https://github.com/kraflab/dsda-doom

NOTE:
The article at the DOOM Wiki declares the forums post to be the website
of the program; therefore I have set the HOMEPAGE variable to it on the
Makefile.

Lucas de Sena.