chore: Update help message

README.md

@@ -26,7 +26,7 @@ protoc --jsonschema_out=. *.proto
 protoc --jsonschema_out=. --jsonschema_opt=output_file_suffix=.yaml *.proto
 ```
 
-#### Shrink bytes for transfer over network.
+#### Shrink bytes for transfer over network
 ```
 protoc --jsonschema_out=. --jsonschema_opt=pretty_json_output=false
 ```

main.go

@@ -11,13 +11,36 @@ import (
 
 var version string = "develop"
 
+const helpMessage = `Usage protoc-gen-jsonschema:
+protoc --jsonschema_out=. (OPTIONS) *.proto
+
+This plugin converts 'protocol buffer' formats to 'json-schema'. Support json-schema 'draft-04, draft-06, draft-07, draft-2019-09, draft-2020-12' versions.
+
+EXAMPLE: I'm not sure which options to use
+protoc --jsonschema_out=. *.proto
+
+EXAMPLE: Generate with yaml format
+protoc --jsonschema_out=. --jsonschema_opt=output_file_suffix=.yaml *.proto
+
+EXAMPLE: Shrink bytes for transfer over network
+protoc --jsonschema_out=. --jsonschema_opt=pretty_json_output=false
+
+EXAMPLE: I'd like to comply with the protobuf JSON mapping standard
+protoc --jsonschema_out=. --jsonschema_opt=int64_as_string=true
+
+Please check https://github.com/pubg/protoc-gen-jsonschema for more details.
+
+FLAGS:
+  --version  : print version
+  --help     : print help
+`
+
 func main() {
 	if len(os.Args) == 2 {
 		if os.Args[1] == "--version" {
 			fmt.Println(version)
 		} else if os.Args[1] == "--help" {
-			fmt.Println("USAGE:")
-			fmt.Println("  protoc-gen-jsonschema --version  : print version")
+			fmt.Print(helpMessage)
 		}
 		return
 	}