docs: the README list of supported operations omits PartiQL and backups #913

The Supported Operations section of README.md lists the operations that ExtendDB serves. On main at 35edb77, the engine serves more operations than the section lists (crates/engine/src/lib.rs, the operation dispatch). The section does not list these:

• PartiQL: ExecuteStatement, BatchExecuteStatement and ExecuteTransaction. feat(partiql): implement ExecuteStatement, BatchExecuteStatement, ExecuteTransaction #411 added them, and docs/differences-from-dynamodb.md lists PartiQL as supported.
• On-demand backups: CreateBackup, DescribeBackup, ListBackups, DeleteBackup and RestoreTableFromBackup.

The engine also dispatches DescribeContinuousBackups, UpdateContinuousBackups and RestoreTableToPointInTime. The upgrade manual says that the server reports point-in-time recovery as unsupported. If the section lists these three operations, it needs that note.

The backend table in #897 refers to this section for the API coverage of PostgreSQL and SQLite.