Fix markdown links in top-level README
Fix markdown links in top-level README

crcrewso/EGSnrc,nrc-cnrc/EGSnrc,marenaud/EGSnrc,nrc-cnrc/EGSnrc,marenaud/EGSnrc,crcrewso/EGSnrc,crcrewso/EGSnrc,nrc-cnrc/EGSnrc,MartinMartinov/EGSnrc,nrc-cnrc/EGSnrc,MartinMartinov/EGSnrc,MartinMartinov/EGSnrc,mchamberland/EGSnrc,ftessier/EGSnrc,MartinMartinov/EGSnrc,mchamberland/EGSnrc,MartinMartinov/EGSnrc,ftessier/EGSnrc,nrc-cnrc/EGSnrc,crcrewso/EGSnrc,crcrewso/EGSnrc,marenaud/EGSnrc,mchamberland/EGSnrc,ftessier/EGSnrc,marenaud/EGSnrc,mchamberland/EGSnrc,marenaud/EGSnrc,marenaud/EGSnrc,mchamberland/EGSnrc,mchamberland/EGSnrc,ftessier/EGSnrc,ftessier/EGSnrc,ftessier/EGSnrc