DOC: add section to sql.rst about overlapping join keys. Close #699
DOC: add section to sql.rst about overlapping join keys. Close #699

koverholt/ibis,wesm/ibis,ibis-project/ibis,cloudera/ibis,wesm/ibis,deepfield/ibis,wesm/ibis,ibis-project/ibis,cpcloud/ibis,cloudera/ibis,laserson/ibis,koverholt/ibis,deepfield/ibis,cloudera/ibis,laserson/ibis,laserson/ibis,deepfield/ibis,cpcloud/ibis,mariusvniekerk/ibis,mariusvniekerk/ibis,koverholt/ibis,ibis-project/ibis,cpcloud/ibis,laserson/ibis,koverholt/ibis,mariusvniekerk/ibis,ibis-project/ibis,cpcloud/ibis,deepfield/ibis,mariusvniekerk/ibis