optipng: add changelog

[RunningDroid]

Testing the changes

• I tested the changes in this PR: YES

xchangelog handles it fine, the file itself starts with non-printing characters* for some reason though:

*: byte sequence is EF BB BF

[classabbyamp]

that's a UTF8 byte-order mark https://en.wikipedia.org/wiki/Byte_order_mark

UTF-8 always has the same byte order, so its only use in UTF-8 is to signal at the start that the text stream is encoded in UTF-8, or that it was converted to UTF-8 from a stream that contained an optional BOM.

[RunningDroid]

Thanks, I had checked that page but stopped reading after it started talking about FE FF.