DOC missing requirements for building documentation

The instructions to build the documentation suggest,

For building the documentation, you will need sphinx, matplotlib and pillow.

yet numpydoc also appear to be a requirement (tested on master with Python 3.6 and sphinx 1.6.3),

make html                                                                                                     [doc-version] 
# These two lines make the build a bit more lengthy, and the
# the embedding of images more robust
rm -rf _build/html/_images
#rm -rf _build/doctrees/
sphinx-build -b html -d _build/doctrees   . _build/html/stable
Running Sphinx v1.6.3

Extension error:
Could not import extension numpydoc (exception: No module named 'numpydoc')
make: *** [Makefile:42: html] Error 1

then if it’s installed,

Extension error:
Could not import extension sphinx_gallery.gen_gallery (exception: No module named 'sphinx_gallery.gen_gallery')
make: *** [Makefile:42: html] Error 1

If confirmed, numpydoc and sphinx-gallery requirements would need to be added to doc/developers/contributing.rst and doc/README.md.

Having e.g. a documentation_requrements.txt somewhere with all the requirements to build the docs (one per line) could also be handy? It could be used from build_tools/circle/build_doc.sh to install the requirements with pip or conda to avoid divergences between the documentation and the CI build in the future…

[glemaitre]

I think that you need to add matplotlib to the doc_requirements.txt. If we go this way, I think that a test_requirements.txt would be nice for pytest, nosetest, and the associated coverage …

IMO, I like the way of the scikit-image fellow (https://github.com/scikit-image/scikit-image/tree/master/requirements) regarding the requirements organisation.

[rth]

Fixed in https://github.com/scikit-learn/scikit-learn/pull/10225