Add mongobeat to list of community beats (elastic#3156) (elastic#3175)

Mongobeat discovers instances in a mongo cluster and can be configured to ship multiple document types - from the commands db.stats() and db.serverStatus()

libbeat/docs/communitybeats.asciidoc

@@ -27,6 +27,7 @@ https://github.com/radoondas/jmxproxybeat[jmxproxybeat]:: Reads Tomcat JMX metri
 https://github.com/mheese/journalbeat[journalbeat]:: Used for log shipping from systemd/journald based Linux systems.
 https://github.com/eskibars/lmsensorsbeat[lmsensorsbeat]:: Collects data from lm-sensors (such as CPU temperatures, fan speeds, and voltages from i2c and smbus).
 https://github.com/consulthys/logstashbeat[logstashbeat]:: Collects data from Logstash monitoring API (v5 onwards) and indexes them in Elasticsearch.
+https://github.com/scottcrespo/mongobeat[mongobeat]:: Monitors MongoDB instances and can be configured to send multiple document types to Elasticsearch.
 https://github.com/adibendahan/mysqlbeat[mysqlbeat]:: Run any query on MySQL and send results to Elasticsearch.
 https://github.com/PhaedrusTheGreek/nagioscheckbeat[nagioscheckbeat]:: For Nagios checks and performance data.
 https://github.com/mrkschan/nginxbeat[nginxbeat]:: Reads status from Nginx.