Tracking issue for keeping a change log (RFC 3)

[kamalmarhubi]

https://github.com/nix-rust/rfcs/blob/master/text/0003-use-changelog.md
nix-rust/rfcs#3

Items from RFC:

• add initial CHANGELOG.md, going back to 0.5.0 per Release procedure and policy #363 (comment)
• add section to CONRTIBUTING.md
• add section to new RELEASE_PROCEDURE.md file

[Susurrus]

I'm going to propose that we leave this at the current 0.5.0 cutoff as this is already almost a year-old issue that hasn't been resolved. As RFC3 doesn't indicate how far back the CHANGELOG should go, just that one is actively maintained, this is also in line with that RFC.