Add a link to the notifications documentation

Summary: Add a link to the noficiations documentation on the "Configuration Guide" article.

Test Plan: Experienced an overwhelming feeling of content.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9727