Mark EC2 SD as stable.

Remove not-yet-released features and add docs for the gce_sd_config

Add prometheus-cpp to client libs

Merge official and external third party exporter-listing

Add code of conduct to Community page

Adding Quobyte to instrumented software

Document remote write config.

Fixes #582
Fixes #583

Add memcached_exporter to downloads page

Fix typos.

Also a couple of non-typos: its vs it's and publically vs publicly.

Add apcupsd exporter to exporters list

Add JustWatch user interview

add databases ClickHouse exporter

Clarify that the in_cluster option only affects discovery.

It wasn't clear to me that the `in_cluster` option for a
`kubernetes_sd_config` only affected authentication for service
discovery, and that to actually scrape the targets, it was still
necessary to explicitly set authentication options like `tls_config` and
`bearer_token_file`, for example, when scraping the Kubernetes
apiserver.

This patch clarifies that in the documentation for `in_cluster`, and
adds a link to the example Prometheus configuration for Kubernetes in
the Prometheus repo that explained this distinction better, and also
provides a useful real-world example of how to monitor a Kubernetes
cluster with Prometheus.

Add BOSH exporter to third-party exporters

Document minute()

optimize png images using zopflipng

Add GCE SD docs

Clarify that rule files are configured via globs