add description

README.md

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 # starlark.vim
-vim plugin for starlark
+[![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
+
+vim plugin for [starlark](https://github.com/bazelbuild/starlark)(syntax only)
+
+## Install
+To install using [Vim-Plug](https://github.com/junegunn/vim-plug):
+```
+" add this line to your .vimrc file
+Plug 'cappyzawa/starlark.vim'
+```