add release notes

**Release Notes**:
```bugfix user github.com/gardener/cert-management #140 @MartinWeindel
Disable followCNAME by default again as it was activated implicitly by github.com/go-acme/lego version upgrade
```
```bugfix operator github.com/gardener/cert-management #138 @MartinWeindel
Fix edge case of inconsistent certificate/secret: request certificate in this case.
```