docs: add badges

Author: mhanberg

README.md

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 # Spitfire
 
+[![Hex.pm](https://img.shields.io/hexpm/v/spitfire)](https://hex.pm/packages/spitfire)
+[![Hex Docs](https://img.shields.io/badge/hex-docs-lightgreen.svg)](https://hexdocs.pm/spitfire/)
+
 Spitfire is a new error resilient parser for Elixir.
 
 The goal of the project is to make it easier for editor tooling to provide code intelligence for incomplete snippets of Elixir code.