added link to the BP heartbeat plugin

Added link to the heartbeat plugin (details here: https://medium.com/@liquideos/increasing-stability-of-the-eos-blockchain-11921477549e)

Some BPs have started using it (as can be seen here: http://heartbeat.liquideos.com/), So people in the community requested that i add it here as well.


A curated list of some more plugins can be found here:
https://github.com/tmuskal/awesome-eosio-plugins

plugins/COMMUNITY.md

@@ -10,6 +10,7 @@ Third parties are encouraged to make pull requests to this file (`develop` branc
 | Kafka | https://github.com/TP-Lab/kafka_plugin |
 | SQL | https://github.com/asiniscalchi/eosio_sql_plugin |
 | ElasticSearch | https://github.com/EOSLaoMao/elasticsearch_plugin |
+| BP Heartbeat  | https://github.com/bancorprotocol/eos-producer-heartbeat-plugin |
 
 ## DISCLAIMER: