DOC - add warning about non-cython c files
DOC - add warning about non-cython c files

Suggestion by Bertrand

alexis-roche/niseg,arokem/nipy,bthirion/nipy,arokem/nipy,nipy/nipy-labs,alexis-roche/register,alexis-roche/niseg,arokem/nipy,alexis-roche/nireg,nipy/nipy-labs,nipy/nireg,alexis-roche/nipy,bthirion/nipy,bthirion/nipy,arokem/nipy,alexis-roche/register,alexis-roche/nipy,alexis-roche/register,bthirion/nipy,alexis-roche/nipy,alexis-roche/nireg,nipy/nireg,alexis-roche/nipy