graphics: Add HELP2 lump.

HELP2 is the screen shown when playing shareware Doom, after
completing the first episode (gives you information on how to buy
the registered version, etc.).  For some reason, ZDoom seems to
expect this lump after completing episode 1, even though
freedoom1.wad isn't a shareware IWAD in any sense. Include HELP2
as a copy of the CREDIT screen; even though it's redundant it
doesn't do any harm.