amp-analytics: describe https requirement for infonline

extensions/amp-analytics/amp-analytics.md

@@ -135,14 +135,14 @@ Adds support for Google Analytics. More details for adding Google Analytics supp
 
 Type attribute value: `infonline`
 
-Adds support for [INFOnline](https://www.infonline.de) / [IVW](http://www.ivw.de). Requires a copy of [amp-analytics-infonline.html](https://3p.ampproject.net/custom/amp-analytics-infonline.html) on a different subdomain than the including AMP file ([why?](https://github.com/ampproject/amphtml/blob/master/spec/amp-iframe-origin-policy.md)). Example: if your AMP files are hosted on `www.example.com`, then `amp-analytics-infonline.html` needs to be on another subdomain such as `iframe.example.com` or `assets.example.com`.
+Adds support for [INFOnline](https://www.infonline.de) / [IVW](http://www.ivw.de). Requires a copy of [amp-analytics-infonline.html](https://3p.ampproject.net/custom/amp-analytics-infonline.html) on a different subdomain than the including AMP file ([why?](https://github.com/ampproject/amphtml/blob/master/spec/amp-iframe-origin-policy.md)). The file must be served via HTTPS. Example: if your AMP files are hosted on `www.example.com`, then `amp-analytics-infonline.html` needs to be on another subdomain such as `iframe.example.com` or `assets.example.com`.
 
 Additionally, the following variables must be defined:
 
 * `st`: Angebotskennung
 * `co`: comment
 * `cp`: code
-* `url`: location of `amp-analytics-infonline.html`
+* `url`: HTTPS location of `amp-analytics-infonline.html`
 
 More details for adding INFOnline / IVW support can be found at [www.infonline.de](https://www.infonline.de/downloads/web-mew-und-ctv/).