doc: add requirements.txt
doc: add requirements.txt

so we can built this project using readthedocs.org

Signed-off-by: Kefu Chai <a4b264ee037c625052534d60505bfd0db4776ac4@redhat.com>

SUSE/teuthology,dmick/teuthology,SUSE/teuthology,ceph/teuthology,ceph/teuthology,ktdreyer/teuthology,dmick/teuthology,dmick/teuthology,SUSE/teuthology,ktdreyer/teuthology