chore: move release notes to CHANGELOG.md (googleapis/google-cloud-cpp-common#128)
chore: move release notes to CHANGELOG.md (googleapis/google-cloud-cpp-common#128)

Part of googleapis/google-cloud-cpp#3334

Note also in this case, the release notes used to be inside the google/cloud directory. In this PR, I'm putting the release notes in a top-level CHANGELOG.md file, and I'm removing the google/cloud/README.md file, which seems to be unnecessary since we split "common" from google-cloud-cpp
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp