Changed Authors to a markdown file, updated the information in it

Author: Kiloku

Authors.md

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+Original source (Freespace 2) by Volition, Inc.; see "`credits.tbl`"
+
+Modifications by the FreeSpace 2 Source Code Project [(FSO SCP)](https://www.hard-light.net/forums/index.php?board=50.0)
+[Project Contributors](https://github.com/scp-fs2open/fs2open.github.com/graphs/contributors)
+
+Special thanks to:
+Volition for making FS2 such a great game
+Dave Baranec for giving us the code and keeping us sanity checked