DOC: Clarified installation process.
DOC: Clarified installation process.

The setup.py file relies on cython and numpy and can't bootstrap. Right
now, if Cython is not installed, it will fallback to
requirements_docs.txt. This makes the docs work on read-the-docs, but
it's an undesirable behavior for end users.

uchicago-cs/deepdish,agarbuno/deepdish,agarbuno/deepdish,uchicago-cs/deepdish