add link to online documentation in README
add link to online documentation in README

rouault/GeoGit,rouault/GeoGit,boundlessgeo/GeoGig,annacarol/GeoGig,markles/GeoGit,markles/GeoGit,rouault/GeoGit,rouault/GeoGit,boundlessgeo/GeoGig,boundlessgeo/GeoGig,markles/GeoGit,markles/GeoGit,annacarol/GeoGig,markles/GeoGit,annacarol/GeoGig