Add background information about the EZV -> STR formats and their history

[Duckwhale]

This ties in with Arcturus via Sonnori (which was claimed to have provided/created the ADP variant used for the audio files), and Amuse World's EZ2DJ series. I haven't had time to review all the sources, nor do I know if it's helpful to actually look into EZV given that there's little point in using them when STR files are available... but it might be worth mentioning since information isn't exactly abundant.

References (reliability and usefulness TBD):

• https://en.wikipedia.org/wiki/EZ2DJ (also check KR/JP etc.)
• http://www.hardcoregaming101.net/korea/part2/company-gravity.htm
• http://www.hardcoregaming101.net/korea/part2/company-deniam.htm (Amuse World)
• http://www.hardcoregaming101.net/korea/part2/company-sonnori.htm
• https://forum.xentax.com/viewtopic.php?t=19103