Improve acceptance test howto
by referencing the k3d setup doc and documenting the
make prepare_ci_environment_k3d target.
## Basic Run
+### Preparation
+
+An accessible Kubernetes instance is required to run the acceptance tests.
+If none is available, K3d can be
+[setup locally](https://docs.epinio.io/installation/install_epinio_on_k3d.html).
+
+The Kubernetes instance needs to be prepared prior to the test run. K3d has
+its own make target for the preparation which can be initiated by running
+`make prepare_ci_environment_k3d`.
+
+### Execution
+
Invoke `make test-acceptance` to running the epinio acceptance tests
with its standard configuration.
