Update CHANGELOG to mention response caching
Update CHANGELOG to mention response caching

Response caching was enabled in PR #635.

RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go