docs: move reference material from the README into docs

The README now covers what keepalive is, a quick start, the adapter table,
and links. Reference material moves to docs/: configuration (every option,
per-adapter keys, validation), deployment (Compose, Coolify, Docker, source),
how it works (writes, retries, timeouts, logging), known issues (PostgreSQL
public-schema permissions and Aiven's ACL key-pattern tilde, by log line),
and adding an adapter (now including ConfigKeys registration).

.gitignore also excludes config.yaml, matching .dockerignore.

docs/adding-an-adapter.md

@@ -0,0 +1,41 @@
+# Adding an adapter
+
+An adapter connects to one datastore, increments a counter on every tick, and releases its resources on close. Adapters live in the `adapter` package, one file each.
+
+1. **Create `adapter/<name>.go`** and implement the `Adapter` interface from [`adapter/adapter.go`](../adapter/adapter.go):
+
+   ```go
+   type Adapter interface {
+   	Connect(ctx context.Context) error           // open the client and create the counter if missing
+   	Increment(ctx context.Context) (int64, error) // add 1 and return the new value
+   	Close(ctx context.Context) error              // release resources; called after every session
+   }
+   ```
+
+2. **Register the factory and its config keys** in `init()`. The factory only reads config and must not do network I/O, because keepalive calls it at startup to validate the file.
+
+   ```go
+   func init() {
+   	ConfigKeys["<name>"] = []string{"url"}
+   	Registry["<name>"] = func(cfg Config) (Adapter, error) {
+   		url, err := cfg.Required("url")
+   		if err != nil {
+   			return nil, err
+   		}
+   		return &myAdapter{url: url, key: cfg.Optional("counter_key", "counter")}, nil
+   	}
+   }
+   ```
+
+   `ConfigKeys` drives the unknown-key warnings; a test fails if an adapter is registered without it. `counter_key` is supplied by the config loader and is not listed.
+
+3. **Follow the conventions** that keep the runner's guarantees:
+   - Honour `ctx` in `Connect` and `Increment`; the runner bounds both.
+   - Store the client on the adapter only after `Connect` succeeds, so `Close` never closes a failed client twice.
+   - Make `Connect` idempotent: it re-runs after every failure and should recreate a missing counter.
+   - Implement `ConnectTimeout() time.Duration` only if setup legitimately needs longer than 1 minute.
+   - Keep the driver pure Go (`CGO_ENABLED=0`).
+
+4. **Document it:** add the adapter to [`config.example.yml`](../config.example.yml), its keys to [Configuration](configuration.md), its writes to [How it works](how-it-works.md), and a row to the README's adapter table.
+
+5. **Test it:** `go vet ./...` and `go test -race ./...`.