Contributor docs: add lightweight guidelines for grouping features

docs/guidelines.md

@@ -260,3 +260,17 @@ Follow these guidelines when setting a `caniuse` value:
   Can I Use isn't perfect.
   Don't use `compute_from` in a way that would not make sense if the corresponding `caniuse` value didn't exist (for example, by pinning support before the introduction of an essential component of the feature).
   In such situations, it's better to comment out the `caniuse` value, make a `TODO` comment, and open an issue about why you did it.
+
+## Groups
+
+The `group` field references one or more groups.
+You can find group definitions in the [`groups/`](../groups/) directory.
+
+Groups are experimental.
+It might not be clear how to group features until more features have been defined.
+
+Don't assign features to two or more groups such that one group is an ancestor of another.
+For example, don't assign a feature to both `css` and `fonts`, since `css` is the parent of `fonts`.
+
+Do assign features to groups when there's an opportunity for future feature composition (see [#971](https://github.com/web-platform-dx/web-features/issues/971)).
+For example, several features for the JavaScript `Array` interface are members of the `array` group.