The Great IWAD Renaming: unique file names for Freedoom

After some discussion and consensus[1], Freedoom's IWADs are now
uniquely named independently of Doom's file names:
 * freedoom1.wad: Phase 1
 * freedoom2.wad: Phase 2

The original intention was that Freedoom would be a complete drop-in
replacement for Doom, so that you can just use Freedoom's doom2.wad
rather than id Software's. What has happened, however, is that people
often desire to have Freedoom installed alongside Doom, and try to
uniquely identify it via filenames for multiplayer servers. This has
resulted in lengthy lists of unofficial IWAD file names implemented by
many engines to locate Freedoom, combined with gamers' personal
opinions on just how they should be renamed.

Even the Debian project, normally not one to modify upstream sources
so drastically, has been renaming our doom2.wad to freedoom.wad,
presumably so that Doom II can be installed simultaneously with
game-data-packager.

Given a free software engine, we can instead standardize on unique
names apart from Doom and get the majority of engines in use changed
over to these ones; users with engines that don't recognize the names
can instead use the near-universal -iwad command-line parameter, or
rename the files back to doom.wad or doom2.wad as needed.

[1] http://www.doomworld.com/vb/freedoom/66856-bikeshedding-renaming-freedooms-iwads/
README: Reflect name changes, use simpler language, talk about FreeDM

Freedoom has grown to no longer focus on any single IWAD anymore, in
addition to becoming more of its own entity than a simple Doom-clone.

Use somewhat simpler language in the README, rather than bombing with
a technical term like IWAD right from the start, in addition to
explaining a bit more about what being Doom-compatible means.

The FreeDM project has spent a bit too much time in the shadows, even
moreso than the more recent Ultimate Doom-compatible IWAD. Give it
some spot light in the README and officially document its special
adherance to vanilla-compatibility.

Finally, lowercase "free software", even the FSF doesn't capitalize it
and we aren't writing in German in this document.