Update README file with kuttl targets

In this patch we update the README.md file to clarify the kuttl related
targets, how they are connected and how they can be used.

Author: Akrog

README.md

@@ -226,14 +226,51 @@ pre-commit run --all-files
 KUTTL (KUbernetes Test TooL) tests validate the operator's behavior in a real
 OpenShift environment.
 
-Before running the tests ensure that:
-- `oc` CLI tool is available in your PATH and you can access an OpenShift cluster
-(e.g., deployed with `crc`) with it
-- The `openshift-lightspeed` namespace is empty or non-existing to prevent collisions
+Kuttl tests are run using the `kuttl-test` make target, which has some
+requirements:
 
-Once you are ready you can run the KUTTL tests using:
+- `kubectl-kuttl`, `diff` and `oc` binaries exist and are in the `PATH`.
+- An OpenShift cluster is up and running (e.g., one deployed with `crc`).
+- `oc` CLI tool can access the OpenShift cluster and is logged in.
+- The OpenStack Lightspeed operator to be tested is installed and running in the
+  OpenShift cluster in the `openstack-lightspeed` namespace.
+
+Using the `kuttl-test` directly is uncommon, as we have 2 helpful targets:
+
+- `kuttl-test-run`: Given a catalog image location deploys OpenStack Lightspeed
+  on the OpenShift cluster, runs the tests (using `kuttl-test`), and removes
+  OpenStack Lightspeed.
+
+- `kuttl-test-ocp`: Builds the operator, bundle and catalog images, pushes them
+   to the OpenShift cluster internal registry, and then runs the kuttl tests
+   (using `kuttl-test-run`).
+
+In both cases it will check that the `kubectl-kuttl` binary is present in the
+system and download it if it's not (target `kuttl`) and both need the
+`openstack-lightspeed` namespace to be empty or non-existing to prevent
+collisions.
+
+For the `kuttl-test-run` target the images need to be available in an image
+registry accessible by the OpenShift cluster.  We can build these images
+ourselves or use images built by others, in any case variable `CATALOG_IMG`
+must point to the catalog image before running `kuttl-test-run`.
+
+Using `kuttl-test-ocp` is useful to build and test everything, but it's too
+wasteful if we are going to run kuttl tests multiple times, where
+`kuttl-test-run` is better as it doesn't rebuild the images on each run.
+
+A useful option when working on kuttl tests, without changes on the operator
+itself, is to use `kuttl-test-ocp` the first time:
+
+```bash
+make kuttl-test-ocp
+```
+
+And then set `CATALOG_IMG` and use the `kuttl-test-run` target in
+consecutive runs:
 
 ```bash
+export CATALOG_IMG=image-registry.openshift-image-registry.svc:5000/openshift-marketplace/operator-catalog:latest
 make kuttl-test-run
 ```