Include README.md in extra_rdoc_files only if present
Include README.md in extra_rdoc_files only if present

Can be deleted when packaging as a vendored gem in an application.
all4miller/mondrian-olap,all4miller/mondrian-olap,rsim/mondrian-olap,jazzido/mondrian-olap,jazzido/mondrian-olap,rsim/mondrian-olap,Contiamo/mondrian-olap,Contiamo/mondrian-olap,jazzido/mondrian-olap