Create Github webhook for Piwik Plugins to release new versions

[mattab]

We brainstormed the easiest and more powerful way for Plugin developers to publish their work on the Marketplace. Using Github allows access to amazing features built-in their platform. We are already using Github extensively in the Marketplace.

Use case

• As a Plugin developer, I want to release a new version of the Plugin.

Steps:

• I create a git tag, commit and push.
• Github WebHook "Piwik Plugins" will forward the tag commit to the Piwik marketplace.
• If the tag is a valid version, and other checks are OK, a new version of the plugin is published.
• if any new version check fails, and if it works, an email is sent to the developer containing feedback on what to do next.

This ticket is about finalizing the Github webhook functionality.

• Update doc at: http://developer.piwik.org/guides/marketplace#setting-up-a-webhook-for-your-github-repository
• maybe we can have text on github updated to "Piwik is an open analytics platform designed to make the most of your data. Piwik be extended with Plugins and Themes. This service notifies the Marketplace(http://plugins.piwik.org) of any new releases to your Piwik plugin. See [http://developer.piwik.org/guides/marketplace](the Publisher Guide) for more information."

[mattab]

The text was updated in github/github-services#740
(not yet deployed in github though)

[mattab]

Fixed doc in matomo-org/developer-documentation@d846067

Nice work! good feeling to be listed on Github in the list of WebHooks.

Now on to the next step: helping get developers started to develop and publish plugins, and inspiring them to do so!