Add hackage badge

Author: diogob

README.md

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-# postgres-websockets [![CircleCI](https://circleci.com/gh/diogob/postgres-websockets.svg?style=svg)](https://circleci.com/gh/diogob/postgres-websockets)
+# postgres-websockets [![CircleCI](https://circleci.com/gh/diogob/postgres-websockets.svg?style=svg)](https://circleci.com/gh/diogob/postgres-websockets) [![Hackage Matrix CI](https://matrix.hackage.haskell.org/api/v2/packages/postgres-websockets/badge)](https://matrix.hackage.haskell.org/package/postgres-websockets)
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 postgres-websockets is a [middleware](https://hackage.haskell.org/package/wai) that adds websockets capabilites on top of [PostgreSQL](https://www.postgresql.org)'s asynchronous notifications using LISTEN and NOTIFY commands.
 The project was largely inspired and originaly designed for use with [PostgREST](https://github.com/begriffs/postgrest).