Kubernetes SD: Remove references to `in_cluster`

`in_cluster` doesn't exist anymore, and the comment for the
`api_server` config option already explains how in-cluster auth works.

Add Marathon SD section to scrape and AM configs.

* Document EC2 profile configuration

* config: document Alertmanager discovery

* config: add alpha notice for Alertmanager SD

I wrote an exporter for gluster fs, its still beta but in active development. 
Check it out at https://github.com/ofesseler/gluster_exporter

Update AlertManager SMTP TLS configuration options

* Added link to Cloudflare exporter

Featured on https://www.wehkamplabs.com/blog/2016/11/06/cloudflare-metrics-using-prometheus/

* Fixed sorting

Fix formatting of k8s-SD config docs

The redcarpet markdown renderer interprets the newlines within bulleted
lists in a different way than GitHub, so the layout is currently broken.

Next release

Add process-exporter to exporter.md

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