Continuous integration/Tutorials/Adding your extension to the shared gate job

There is a job that runs tests for various actively maintained extensions against MediaWiki core patches and those extensions. The goal is to test extension compatibility with core and extensions, and prevent breaking changes in core affecting widely used extensions.

Some details that should be used when creating this page:
 * https://lists.wikimedia.org/pipermail/wikitech-l/2015-January/080098.html
 * https://phabricator.wikimedia.org/T86930#980523
 * Requests_for_comment/Extensions_continuous_integration