Add Data for PM Handbook link

.gitlab/issue_templates/Snowplow event tracking.md

@@ -4,6 +4,7 @@
 * Snowplow is currently not used for self-hosted instances of GitLab - Self-hosted still rely on usage ping for product analytics - Snowplow is used for GitLab SaaS
 * You do not need to create an issue to track generic front-end events, such as All page views, sessions, link clicks, some button clicks, etc.
 * What you should capture are specific events with defined business logic. For example, when a user creates an incident by escalating an existing alert, or when a user creates and pushes up a new Node package to the NPM registry.
+* For more details read https://about.gitlab.com/handbook/business-ops/data-team/programs/data-for-product-managers/
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