Converting HTML to Markdown Using RSS

I would like to move away from CMS that I'm using to GitHub pages. I think that the easiest way is to use my RSS feed as source, but I don't know how to convert. I get it that not everything can be converted, but just to manage to get the images and text out and formated right it would be great.

My second option is to copy HTML source code, but I don't think that GitHub pages support all of the HTML tags.