Add docs

docs/preview.md

@@ -46,3 +46,20 @@ preview = true
 Or, if you provided the `--preview` CLI flag.
 
 To see which rules are currently in preview, visit the [rules reference](https://beta.ruff.rs/docs/rules/).
+
+## Selecting single preview rules
+
+When preview mode is enabled, selecting rule categories or prefixes will include all preview rules that match.
+If you would prefer to opt-in to each preview rule individually, you can toggle the `explicit-preview-rules`
+setting in your `pyproject.toml`:
+
+```toml
+[tool.ruff]
+preview = true
+explicit-preview-rules = true
+```
+
+In our previous example, `--select` with `ALL` `HYP`, `HYP0`, or `HYP00` would not enable `HYP001`. Each preview
+rule will need to be selected with its exact code, e.g. `--select ALL,HYP001`.
+
+If preview mode is not enabled, this setting has no effect.