feat: add the buf completion script to the same structure as in deno or gh plugins

plugins/buf/README.md

@@ -0,0 +1,9 @@
+# Buf plugin
+
+This plugin adds completion for [Buf CLI](https://github.com/bufbuild/buf), a tool working with Protocol Buffers.
+
+To use it, add `buf` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... buf)
+```