Currently master is not added and this may cause some build to fail.
See the building of the staging hibernate.org website.

There is an example about it on this page:

- name: ensure github.com is a known host
  lineinfile:
    dest: /root/.ssh/known_hosts
    create: yes
    state: present
    line: "{{ lookup('pipe', 'ssh-keyscan -t rsa github.com') }}"
    regexp: "^github\\.com"

This is just an example that I've found on the web. I haven't try it yet