DOC: add guidelines for building the docs
DOC: add section contributing to pandas

+ link to README.rst in /docs
Merge pull request #5996 from jorisvandenbossche/doc-guide

DOC: add guidelines for building the docs
BLD/DOC: report context on warnings and add :okwarning: to ipython_directive
Merge pull request #6082 from y-p/PR_ipython_directive

BLD/DOC: report context on warnings and add :okwarning: to ipython_directive
DOC/BLD: ipython_directive cleanups
DOC: minor doc changes
BUG: not converting scalars properly to M8/m8 on assignment (GH6079)
CLN: removed need for SNDArray ; simplied SeriesGroupber / SeriesBinGrouper