move development mailing list info to Contributing section

The version expressed by the AlertManager was changed in the following
commit: 
  https://github.com/prometheus/alertmanager/commit/3269bc39e121452abdce24db1d3d9003d99184d1

This updates the documentation to reflect that change. 

* Suffix the groupKey with a `,` character

`2 * 3 ^ 2` being equivalent to `2 * (3 ^ 2)` doesn't actually demonstrate that `^` is right associative. It just follows from operator precedence. The significance of right associativity is better exemplified by something of same precedence level as `^`.

derived two orange logos from the original grey logo

Currently, there is a large set of exporters available for a lot of different software tools.
However, the popular runtime "PHP-FPM", used for executing PHP script through the
FastCGI protocol, is not listed.

Various exporters currently exist, and the author has not tested any (including the
suggested) extensively. However, a quick review of the available options shows that,
in the authors limited review, this appears to be well constructed, thought out and
documented.

This commit adds the exporter to the canonical list for easy discovery. 

Design Notes:

Alternatives considered were:
- https://github.com/peakgames/php-fpm-prometheus/ (no releases, less docs)
- https://github.com/blablacar/phpfpm-prometheus-exporter (no docs)

* Shift the PHP-FPM exporter to the "misc" section

Previously, the author had added a new section titled "interpreters", and
added the PHP-FPM exporter to this section under the belief that, given there
was none (that is, neither node or python are represented) this could be a
separate class of application. However, with review it looks like the better
place for this exporter is under the "misc" section.

This commit moves the PHP-FPM exporter to this section.

These labels were added in:
https://github.com/prometheus/prometheus/pull/2506

PagerDuty has a page specifically for integrating
with Prometheus/Alertmanager, which seems like a
good idea to link to from our documentation.

Add CRG to directly instrumented software

Add Google Stackdriver & Shield exporters to the list of exporters

Document our security model

Include some aspects that may be non-obvious
to a casual user.

Fixes #715

blog: Announce alpha3 and new rule format.

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

See https://github.com/prometheus/alertmanager/blob/master/config/notifiers.go#L238

1.7 Release

Added OpenStack service discovery docs.

This reverts commit b66ec14a.

The PromCon 2017 CfP is closed now.