Merge pull request #128 from prometheus/haskell-client-lib

Mention Haskell client library

content/docs/instrumenting/clientlibs.md

@@ -23,6 +23,7 @@ Unofficial third-party client libraries:
 * [Node.js](https://github.com/StreamMachine/prometheus_client_nodejs)
 * [.NET / C#](https://github.com/andrasm/prometheus-net)
 * [Bash](https://github.com/aecolley/client_bash)
+* [Haskell](https://github.com/fimad/prometheus-haskell)
 
 When Prometheus scrapes your instance's HTTP endpoint, the client library
 sends the current state of all tracked metrics to the server.