Add note to playwright docs about transformDOM

This question came up recently and it seems like we should document it
to help point people in the right direction.

More info:

happo/happo-playwright#5

Author: lencioni

Diff for: docs/playwright.md

@@ -307,3 +307,11 @@ happened:
   element, never the surrounding page.
 - There could be a bug in how `happo-e2e` collects styles and assets. Reach out
   to [email protected] and we'll triage.
+
+### I'm migrating from `happo-cypress` and `transformDOM` is not working
+
+The `transformDOM` option is currently not supported in `happo-playwright`.
+
+If you're migrating from `happo-cypress` and are using `transformDOM` to hide
+content, you may be able to achieve similar results using
+[`data-happo-hide`](./hiding-content.md).