Add initial e2e tests

test/README.md

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+# End-to-end tests
+
+## With [kind](https://kind.sigs.k8s.io/)
+
+[`kind`](https://kind.sigs.k8s.io/) and `kubectl` are automatically downloaded
+except if `SKIP_INSTALL=true` is set.
+A `kind` cluster is automatically created before the tests, and deleted after
+the tests.
+The `prometheus-adapter` container image is build locally and imported
+into the cluster.
+
+```bash
+KIND_E2E=true make test-e2e
+```
+
+## With an existing Kubernetes cluster
+
+If you already have a Kubernetes cluster, you can use:
+
+```bash
+KUBECONFIG="/path/to/kube/config" REGISTRY="my.registry/prefix" make test-e2e
+```
+
+- The cluster should not have a namespace `prometheus-adapter-e2e`.
+  The namespace will be created and deleted as part of the E2E tests.
+- `KUBECONFIG` is the path of the [`kubeconfig` file].
+  **Optional**, defaults to `${HOME}/.kube/config`
+- `REGISTRY` is the image registry where the container image should be pushed.
+  **Required**.
+
+[`kubeconfig` file]: https://kubernetes.io/docs/concepts/configuration/organize-cluster-access-kubeconfig/
+
+## Additional environment variables
+
+These environment variables may also be used (with any non-empty value):
+
+- `SKIP_INSTALL`: skip the installation of `kind` and `kubectl` binaries;
+- `SKIP_CLEAN_AFTER`: skip the deletion of resources (`Kind` cluster or
+  Kubernetes namespace) and of the temporary directory `.e2e`;
+- `CLEAN_BEFORE`: clean before running the tests, e.g. if `SKIP_CLEAN_AFTER`
+  was used on the previous run.