README: add Layering section pointing to higher-level wrappers

ISSUE-29 — explain when to use dqlite-client directly vs dqlite-dbapi
or sqlalchemy-dqlite, so newcomers don't mistake the wire client
for the ergonomic interface.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

README.md

@@ -37,6 +37,20 @@ async with pool.acquire() as conn:
     rows = await conn.fetch("SELECT 1")
 ```
 
+## Layering
+
+`dqlite-client` is the low-level async wire client. Most applications
+should reach for one of the higher-level packages instead:
+
+- **`dqlite-dbapi`** — PEP 249–compliant wrapper (sync *or* async).
+  Plug-and-play with SQLAlchemy, Alembic, most ORMs.
+- **`sqlalchemy-dqlite`** — SQLAlchemy 2.0 dialect, built on top of
+  `dqlite-dbapi`.
+
+Use `dqlite-client` directly when you need fine-grained control over
+the wire protocol: custom cluster bootstrapping, explicit message
+decoding, or building a new high-level driver.
+
 ## Development
 
 See [DEVELOPMENT.md](DEVELOPMENT.md) for setup and contribution guidelines.