docs: add debugging tips to DEVELOPMENT.md

DEVELOPMENT.md

@@ -77,4 +77,44 @@ Deploy `notification-controller` into the cluster that is configured in the loca
 
 ```sh
 make deploy
-```
+```
+
+## Debugging the controller locally
+
+When reproducing an issue or stepping through reconciliation logic, the
+following knobs make local runs cheaper and the resulting logs easier to
+read.
+
+### Limit the watched namespace
+
+The controller watches every namespace by default. To narrow it to a single
+namespace, set the `RUNTIME_NAMESPACE` environment variable before invoking
+`make run`:
+
+```sh
+RUNTIME_NAMESPACE=flux-system make run
+```
+
+### Reduce reconcile concurrency
+
+Each `Alert`, `Provider`, and `Receiver` reconcile is processed concurrently
+(default `--concurrent=4`). When debugging it is almost always easier to
+follow a serial trace; pass `--concurrent=1` so reconciles run one at a
+time:
+
+```sh
+go run ./main.go --concurrent=1
+```
+
+### Suspend unrelated objects
+
+If the controller is sharing a cluster with other Flux objects, suspend
+anything not relevant to the test you're running so their reconciles don't
+interleave with yours:
+
+```sh
+flux suspend alert <name>
+flux suspend receiver <name>
+```
+
+Resume them with `flux resume` when you're done.