Update the readme

[icculus]

The Readme should be in Markdown format and otherwise modernized.

[dg0yt]

It would be clear to verify the copyright. The README reads like GPL-2.0-only, but the source files indicate GPL-2.0-or-later.

[chenrui333]

It would be clear to verify the copyright. The README reads like GPL-2.0-only, but the source files indicate GPL-2.0-or-later.

just noticed the same, it would be good to correct that before making the markdown doc

[icculus]

The README has been moved to Markdown and now explicitly notes that the license is LGPL 2.1 or later.

[icculus]

This at least needs the broken links updated or removed before we close this specific issue, though.