x509-cert: Changelog is empty for `0.3.0-pre.0`

I'm having a hard time figuring out how to migrate from x509-cert 0.2 to x509-cert 0.3.0-pre.0; the latter was published with no changelog entries. I have now spent hours digging through its source code and the PRs here attempting to figure out what the intentions behind various changes were, and how I'm supposed to handle things now.

The latest thing I just spent time figuring out (but not the first or only thing): x509_cert::builder::Profile was previously an enum, and Profile::Manual (which opted out of the default extensions) is used in the yubikey crate for self-signed certificates. The public API now has that type as a trait, and there are implementations of the trait for some specific use cases, but I cannot find any implementation that matches the previous behaviour of Profile::Manual.

It would be really helpful if the changelog was kept up-to-date as PRs are made.

Start by comparing the x509-cert 0.2 and 0.3.0-pre.0 source and reviewing the related pull requests mentioned in the issue. Document the breaking changes and migration guidance, including the replacement for Profile::Manual used by the yubikey crate; done means the changelog explains the release changes and affected upgrade paths.