clarify iframe usage for ads (#3787)

3p/README.md

@@ -30,7 +30,7 @@ Examples: Youtube, Vimeo videos; Tweets, Instagrams; comment systems; polls; qui
 - We welcome pull requests by all ad networks for inclusion into AMP.
 - All ads and all sub resources must be served from HTTPS.
 - Must play well within [AMP's sizing framework](https://github.com/ampproject/amphtml/blob/master/spec/amp-html-layout.md).
-- Direct iframe embeds not using our 3p iframe mechanism (used by most ads) are preferred.
+- For display ads support, always implement amp-ad and instruct your client to use your amp-ad implementation instead of using amp-iframe. Althought amp-iframe will render the ad, ad clicks will break and viewability information is not available.
 - Providing an optional image only zero-iframe embed is appreciated.
 - Support viewability and other metrics/instrumentation as supplied by AMP (via postMessage API)
 - Try to keep overall iframe count at one per ad. Explain why more are needed.

ads/README.md

@@ -3,7 +3,7 @@
 See also our [ad integration guidelines](../3p/README.md#ads) and [3rd party ads integration guidelines](./integration-guide.md)
 
 ## Overview
-Ads are just another external resource and must play within the same constraints placed on all resources in AMP. We aim to support a large subset of existing ads with little or no changes to how the integrations work. Our long term goal is to further improve the impact of ads on the user experience through changes across the entire vertical client side stack.
+Ads are just another external resource and must play within the same constraints placed on all resources in AMP. We aim to support a large subset of existing ads with little or no changes to how the integrations work. Our long term goal is to further improve the impact of ads on the user experience through changes across the entire vertical client side stack. Although technically feasible, do not use amp-iframe to render display ads. Using amp-iframe for display ads breaks ad clicks and prevents recording viewability information.
 
 ## Constraints
 A summary of constraints placed on external resources such as ads in AMP HTML: