Add badges to README: Clojars, CircleCI, Slack

Signed-off-by: Christian Weilbach <christian@weilbach.name>

Author: whilo

README.md

@@ -1,5 +1,9 @@
 # Stratum
 
+[![Clojars Project](https://img.shields.io/clojars/v/org.replikativ/stratum.svg)](https://clojars.org/org.replikativ/stratum)
+[![CircleCI](https://circleci.com/gh/replikativ/stratum.svg?style=shield)](https://circleci.com/gh/replikativ/stratum)
+[![Slack](https://img.shields.io/badge/slack-join_chat-brightgreen.svg)](https://clojurians.slack.com/archives/CB7GJAN0L)
+
 *Stratum is in beta. Please provide feedback and report any issues you run into.*
 
 SIMD-accelerated SQL engine for the JVM. Every table is a branchable, copy-on-write value.