📝 Add Codecov badge

Author: lambdalisue

README.md

@@ -3,6 +3,7 @@
 [![deno land](http://img.shields.io/badge/available%20on-deno.land/x-lightgrey.svg?logo=deno)](https://deno.land/x/vim_channel_command)
 [![deno doc](https://doc.deno.land/badge.svg)](https://doc.deno.land/https/deno.land/x/vim_channel_command/mod.ts)
 [![Test](https://github.com/vim-denops/deno-vim-channel-command/workflows/Test/badge.svg)](https://github.com/vim-denops/deno-vim-channel-command/actions?query=workflow%3ATest)
+[![codecov](https://codecov.io/github/vim-denops/deno-vim-channel-command/branch/main/graph/badge.svg?token=FYH6RHO6UC)](https://codecov.io/github/vim-denops/deno-vim-channel-command)
 
 [Deno][deno] module to support [Vim's Channel commands][vim's channel commands].