doc: Add wallet release notes for 0.21.0
doc: Add wallet release notes for 0.21.0

Former-commit-id: c3de0cfdb5b6c738f178da84f24cb4f7a723cdc0
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin