remove or update CHANGELOG.md

[mdoering]

https://github.com/gbif/gbif-api/blob/master/CHANGELOG.md has only 3 entries and stops at version 0.57 in 2017

[MattBlissett]

Documenting our change policy is probably more useful.

e.g. that we add enumeration values and object properties while keeping the same version, so clients need to be prepared for this.

Although that would still belong in the API documentation on GBIF.org, with a pointer from here.