Facebook instant article

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Hi all,

I don't see any add-on that enables to create the good RSS feed for Facebook instant article.
After investigation, I just need to add the element <content:encoded> in the current feed proposed by concrete5. This tag should enclose the content of the page in a CDATA section.
I have found that feeds are generated from class concrete\src\Page\Feed.php but I don't know how to get the missing data (content of my page).
Could you help me to setup this ?

Many many thanks,
Matthieu