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.