Fix `post` values in result

Improve k8 discovery docs.

Fix formatting.
Remove long example from reference documentation, point off
to existing example instead.

Remove digitalocean/rsyslog_exporter from lost of exporters.

Closes #440

Add best pratice around operation counters

Add link to zookeeper_exporter

Replace the deprecated third-party memcached_exporter by the official one

Add yet another RabbitMQ Exporter

Remove leftover from old  behavior

Next release

Update docs for k8s pods sd

Add docs for ignoring and new group_* semantics.

re-order exporters list

So the differences with listed one (https://github.com/kbudde/rabbitmq_exporter) are these:
 - prometheus_rabbitmq_exporter is not standalone daemon but a plugin for RabbitMQ itself. 
 - it also exports Erlang VM stats.

Also I tried to stay compatible with rabbitmq_exporter as much as possible (probably added a couple of labels)

List unofficial Erlang client

Aerospike exporter

Adding BIG-IP exporter

Add unless to precedence table

Document holt_winters()

Fixes #363

Adding in link to Rancher exporter to docs