document how badges work

doc/development/doc_styleguide.md

@@ -157,6 +157,41 @@ below.
 
     Otherwise, leave this mention out.
 
+### Product badges
+
+When a feature is available in EE-only tiers, add the corresponding tier according to the
+feature availability:
+
+- For GitLab Starter and GitLab.com Bronze: `** [STARTER] **`
+- For GitLab Premium and GitLab.com Silver: `** [PREMIUM] **`
+- For GitLab Ultimate and GitLab.com Gold: `** [ULTIMATE] **`
+- For GitLab Core and GitLab.com Free: `** [CORE] **`
+
+To exclude GitLab.com tiers (when the feature is not available in GitLab.com), add the
+keyword "only":
+
+- For GitLab Starter: `** [STARTER ONLY] **`
+- For GitLab Premium: `** [PREMIUM ONLY] **`
+- For GitLab Ultimate: `** [ULTIMATE ONLY] **`
+- For GitLab Core: `** [CORE ONLY] **`
+
+The tier should be ideally added to headers, so that the full badge will be displayed.
+But it can be also mentioned from paragraphs, list items, and table cells. For these cases,
+the tier mention will be represented by an orange question mark.
+E.g. `** [STARTER] **` renders **[STARTER]**.
+
+The absence of tiers' mentions mean that the feature is available in GitLab Core,
+GitLab.com Free, and higher tiers.
+
+Note that spaces between `*` and `[]` were added for escaping the special markup.
+
+#### How it works
+
+Introduced by [!244](https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/244),
+the special markup `** [STARTER] **` will generate a `span` element to trigger the
+badges and tooltips (`**[STARTER]**`). When the keyword "only" is added, the
+corresponding GitLab.com badge will not be displayed.
+
 ### GitLab Restart
 
 There are many cases that a restart/reconfigure of GitLab is required. To