Add basic badges to README

I always like at least these 2 to have easy access to the most current version number and the docs. Can also add more (see benchee 😁 ) but didn't wanna overdo it.

README.md

@@ -1,4 +1,4 @@
-# Jason
+# Jason [![Hex Version](https://img.shields.io/hexpm/v/jason.svg)](https://hex.pm/packages/jason) [![Hex Docs](https://img.shields.io/badge/docs-hexpm-blue.svg)](https://hexdocs.pm/jason/)
 
 A blazing fast JSON parser and generator in pure Elixir.