Document deferred using sphinxcontrib-emacs

[tkf]

This is the PR to track migration of deferred documentation to sphinxcontrib-emacs
https://github.com/flycheck/sphinxcontrib-emacs

TODO before pull:

• Setup readthedocs (RTD). Set build branch to doc (this branch)
• Move sample code etc. in README to doc/source/
• Wait until markup of sphinxcontrib-emacs is settled (probably until it is relased?: https://github.com/flycheck/sphinxcontrib-emacs/issues/3)
• Copy document README.markdown to docstrings in deferred.el and concurrent.el.
• Check that everything works in RTD, then change the build branch to master.
• Then pull!

[tkf]

http://emacs-deferred.readthedocs.org

[tkf]

I added "How to build document"
http://emacs-deferred.readthedocs.org/en/latest/development.html#how-to-build-document

[AdrieanKhisbe]

$ touch this :)