Add review guidelines for Snowplow

doc/development/snowplow/review_guidelines.md

+---
+stage: Growth
+group: Product Intelligence
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Snowplow review guidelines
+
+This page includes introductory material for a
+[Product Intelligence](https://about.gitlab.com/handbook/engineering/development/growth/product-intelligence/)
+review, and is specific to Snowplow related reviews. For broader advice and
+general best practices for code reviews, refer to our [code review guide](../code_review.md).
+
+## Resources for reviewers
+
+- [Snowplow Guide](index.md)
+- [Event Dictionary](dictionary.md)
+
+## Review process
+
+We recommend a Product Intelligence review when an MR involve changes in
+events or touches Snowplow related files.
+
+### Roles and process
+
+#### The merge request **author** should
+
+- For frontend events, when relevant, add a screenshot of the event in
+  the [testing tool](../snowplow/index.md#developing-and-testing-snowplow) used.
+- For backend events, when relevant, add the output of the
+  [Snowplow Micro](index.md#snowplow-mini) good events
+  `GET http://localhost:9090/micro/good` (it might be a good idea
+  to reset with `GET http://localhost:9090/micro/reset` first).
+- Update the [Event Dictionary](event_dictionary_guide.md).
+
+#### The Product Intelligence **reviewer** should
+
+- Check that the [event taxonomy](../snowplow/index.md#structured-event-taxonomy) is correct.
+- Check the [usage recommendations](../snowplow/index.md#usage-recommendations).
+- Check that the [Event Dictionary](event_dictionary_guide.md) is up-to-date.
+- If needed, check that the events are firing locally using one of the
+[testing tools](../snowplow/index.md#developing-and-testing-snowplow) available.
+- Approve the MR, and relabel the MR with `~"product intelligence::approved"`.