So we can use this code to insert some front matter before exporting org file to HTML, specifically for Jekyll. 

(defun get-org-buffer-title ()
  "Get the title of the current org buffer from #+title."
  (with-current-buffer (current-buffer)
    (let ((ast (org-element-parse-buffer 'greater-element)))
      (org-element-map ast '(keyword)
        (lambda(kw) (plist-get (cadr kw) :value))
        nil
        t))))

(defun add-post-frontmatter (backend)
  "BACKEND is the export back-end being used, as a symbol."
  (if (org-export-derived-backend-p backend 'html)
      (with-current-buffer (current-buffer)
        (goto-char (re-search-forward ":END:"))
        (insert (concat "\n#+begin_export html\n---\nlayout: post\ntitle: \""
                        (get-org-buffer-title)
                        "\"\nexcerpt:\n---\n#+end_export\n")))))

(add-hook 'org-export-before-parsing-hook #'add-post-frontmatter)