Add README

Author: garyb

README.md

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+# purescript-json
+
+[![Build status](https://github.com/purescript/purescript-json/workflows/CI/badge.svg?branch=master)](https://github.com/purescript/purescript-json/actions?query=workflow%3ACI+branch%3Amaster)
+
+Standard types for JSON and JSON objects, and basic operations for working with them.
+
+For efficiency and performance reasons this library provides an interface for working with JSON without using PureScript ADTs, and instead operates on the underlying representation.
+
+## Differences from Argonaut
+
+This library is similar to the traditionally used `argonaut-core` library, but has been implemented with an eye to making it backend agnostic. As such, it does not use `Foreign.Object` as the representation for JSON objects, and instead provides its own type.