Add documentation about passing custom flags

examples/custom_flags/README.md

@@ -0,0 +1,29 @@
+# Using custom flags
+
+You can pass additional custom flags to the CLI
+using the `flag` package and defining the custom flags before calling `envconf.NewFromFlags()`.
+
+For example:
+
+```console
+❯ go test -v ./... -args --my-custom-flag hello
+=== RUN   TestWithCustomFlag
+=== RUN   TestWithCustomFlag/feature
+=== RUN   TestWithCustomFlag/feature/custom_flag
+    custom_flags_test.go:52: Custom flag my-custom-flag: hello
+--- PASS: TestWithCustomFlag (0.00s)
+    --- PASS: TestWithCustomFlag/feature (0.00s)
+        --- PASS: TestWithCustomFlag/feature/custom_flag (0.00s)
+PASS
+ok   sigs.k8s.io/e2e-framework/examples/custom_flags 0.491s
+```
+
+or by compiling the test code
+
+```console
+❯ go test -c -o custom_flags.test .
+❯ ./custom_flags.test --help 2>&1 | grep -A2 my-custom
+  -my-custom-flag string
+     my custom flag for my tests
+  -namespace string
+```

examples/custom_flags/custom_flags_test.go

@@ -0,0 +1,56 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package custom_flags
+
+import (
+	"context"
+	"flag"
+	"os"
+	"testing"
+
+	log "k8s.io/klog/v2"
+
+	"sigs.k8s.io/e2e-framework/pkg/env"
+	"sigs.k8s.io/e2e-framework/pkg/envconf"
+	"sigs.k8s.io/e2e-framework/pkg/features"
+)
+
+var (
+	test         env.Environment
+	myCustomFlag string
+)
+
+func TestMain(m *testing.M) {
+	flag.StringVar(&myCustomFlag, "my-custom-flag", "", "my custom flag for my tests")
+	// create config from flags (always in TestMain because it calls
+	// flag.Parse())
+	cfg, err := envconf.NewFromFlags()
+	if err != nil {
+		log.Fatalf("failed to build envconf from flags: %s", err)
+	}
+	test = env.NewWithConfig(cfg)
+	os.Exit(test.Run(m))
+}
+
+func TestWithCustomFlag(t *testing.T) {
+	f := features.New("feature")
+	f.Assess("custom flag", func(ctx context.Context, t *testing.T, _ *envconf.Config) context.Context {
+		t.Logf("Custom flag my-custom-flag: %v", myCustomFlag)
+		return ctx
+	})
+	test.Test(t, f.Feature())
+}