add helpText guidelines (#2225)

CONTRIBUTING.md

@@ -33,6 +33,25 @@ If you have a contribution for our [documentation](https://developers.google.com
 1. Ensure that your code adheres to the existing style in the sample to which you are contributing.
 1. Submit a pull request.
 
+## helpText guidelines
+
+Keep the `helpText` of an audit as short as possible. When a reference doc for the audit exists on
+developers.google.com/web, the `helpText` should only explain *why* the user should care
+about the audit, not *how* to fix it.
+
+Do:
+
+    Serve images that are smaller than the user's viewport to save cellular data and
+    improve load time. [Learn more](https://developers.google.com/web/tools/lighthouse/audits/oversized-images).
+
+Don't:
+
+    Serve images that are smaller than the user's viewport to save cellular data and
+    improve load time. Consider using responsive images and client hints.
+
+If no reference doc exists yet, then you can use the `helpText` as a stopgap for explaining
+both why the audit is important and how to fix it.
+
 # For Maintainers
 
 ## Updating traceviewer source