document HASHFILE option in using.rst
document HASHFILE option in using.rst

airspeed-velocity/asv,airspeed-velocity/asv,spacetelescope/asv,qwhelan/asv,spacetelescope/asv,qwhelan/asv,airspeed-velocity/asv,spacetelescope/asv,qwhelan/asv,qwhelan/asv,airspeed-velocity/asv,spacetelescope/asv