Contributing docs: add hints on lumping and splitting

docs/CONTRIBUTING.md

@@ -32,6 +32,31 @@ For example, the `fetch()` API is a feature, the `:has()` CSS pseudo-class funct
 
 For the time being, this repository focuses only on features that are implemented in some browser and that are already documented on MDN.
 
+### Hints for distinguishing features
+
+Sometimes it can be hard to tell whether something (like an HTML attribute or JavaScript method) is a distinct feature or part of another feature.
+Here are some questions to ask yourself when deciding whether something is a distinct feature of the web platform:
+
+- Is there evidence that developers recognize it as a feature?
+  Blog posts, Stack Overflow questions, and social media posts by developers about the feature are good places to learn whether developers see something as part of a category or as standing alone
+  (they're also a good source of other information, such as a name for the feature).
+
+- Do existing publications already describe this as a distinct feature?
+  For example, an original Can I Use entry (or request for one on [Fyrd/caniuse](https://github.com/Fyrd/caniuse/issues)) is a good sign that developers will recognize this as a feature.
+  Articles in developer-audience publications (such as CSS-Tricks, DEV.to, and sometimes MDN) are also helpful indicators.
+
+- Do you _expect_ developers to recognize it as a feature?
+  Very new features are harder to sort out because developers don't know about them already.
+  In such cases, you have to make educated guesses.
+  Sometimes you can use prior features' development and specification process to make good guesses.
+  For example, TC39 proposals often emerge as distinct features, while CSS specifications often contain several distinct but related features or incremental additions to existing features.
+  Sometimes you may need to talk to developers or subject matter experts before making a decision.
+
+Creating features is more art than science.
+You won't always get it right.
+That's OK!
+Features can be renamed, split, or merged later.
+
 ### Feature file name and format
 
 Features in this repository are authored as **YAML files** that are stored in the [`features`](../features) directory. Each file corresponds to a single feature, and contains metadata about the feature, such as its name, description, specification, or support data.