Update README.md

Author: kijimad

README.md

@@ -2,6 +2,7 @@
 [![Build](https://github.com/kijimaD/textlint-plugin-org/actions/workflows/build.yml/badge.svg)](https://github.com/kijimaD/textlint-plugin-org/actions/workflows/build.yml)
 [![test](https://github.com/kijimaD/textlint-plugin-org/actions/workflows/test.yml/badge.svg)](https://github.com/kijimaD/textlint-plugin-org/actions/workflows/test.yml)
 [![lint](https://github.com/kijimaD/textlint-plugin-org/actions/workflows/lint.yml/badge.svg)](https://github.com/kijimaD/textlint-plugin-org/actions/workflows/lint.yml)
+[![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 # textlint-plugin-org
 
 Add Org mode support for [textlint](https://github.com/textlint/textlint "textlint").