DOC: add docstring for geodataframe.py merge function (#1176)
DOC: add docstring for geodataframe.py merge function (#1176)

* DOC: add docstring for geodataframe.py merge function

* Fix underline lengths to match headlines

jdmcbr/geopandas,jorisvandenbossche/geopandas,jorisvandenbossche/geopandas,geopandas/geopandas,jorisvandenbossche/geopandas,jdmcbr/geopandas,geopandas/geopandas,geopandas/geopandas