[CHORE] Add release notes, bump minor version.
Merge pull request #129 from ripple/release-3.21.0-rc1

Release 3.21.0 rc1
Merge branch 'develop' into release-3.21.0-rc1
Merge pull request #130 from ripple/release-3.21.0-rc1

Release 3.21.0 rc1
[FIX] Record to_address_id not from_address_id.
[CHORE] cli fund_hot_wallet say cold_wallet_secret not secret.
[TASK] Add release notes and bump patch version.
Merge pull request #142 from ripple/release-3.21.1-rc2

Release 3.21.1 rc2
Merge branch 'master' into develop
Merge branch 'develop' of github.com:ripple/gatewayd into develop