Add description and example for nested value

Signed-off-by: joshuabezaleel <joshua.bezaleel@gmail.com>

cmd/porter/bundle.go

@@ -61,7 +61,7 @@ Porter uses the docker driver as the default build driver, an alternate driver m
   porter build --version 0.1.0
   porter build --file path/to/porter.yaml
   porter build --dir path/to/build/context
-  porter build --custom version=0.2.0
+  porter build --custom version=0.2.0 --custom myapp.version=0.1.2
 `,
 		PreRunE: func(cmd *cobra.Command, args []string) error {
 			return opts.Validate(p)
@@ -83,7 +83,7 @@ Porter uses the docker driver as the default build driver, an alternate driver m
 	f.StringVar(&opts.Driver, "driver", porter.BuildDriverDefault,
 		fmt.Sprintf("Experimental. Driver for building the invocation image. Allowed values are: %s", strings.Join(porter.BuildDriverAllowedValues, ", ")))
 	f.StringSliceVar(&opts.Customs, "custom", nil,
-		"Define an individual key-value pair for custom section in the form of NAME=VALUE. May be specified multiple times.")
+		"Define an individual key-value pair for the custom section in the form of NAME=VALUE. Use dot notation to specify a nested custom field. May be specified multiple times.")
 
 	// Allow configuring the --driver flag with build-driver, to avoid conflicts with other commands
 	cmd.Flag("driver").Annotations = map[string][]string{