Add note about the project status (#10)

Author: wata727

README.md

@@ -3,6 +3,8 @@
 [![GitHub release](https://img.shields.io/github/release/terraform-linters/tflint-ruleset-terraform.svg)](https://github.com/terraform-linters/tflint-ruleset-terraform/releases/latest)
 [![License: MPL 2.0](https://img.shields.io/badge/License-MPL%202.0-blue.svg)](LICENSE)
 
+**This ruleset is a work in progress. Not ready for use yet.**
+
 TFLint ruleset plugin for Terraform Language
 
 This ruleset focus on possible errors and best practices about Terraform Language.