doc: Add coveralls badge
doc: Add coveralls badge

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

dlespiau/proxy,dlespiau/clearcontainers-proxy,clearcontainers/proxy,clearcontainers/proxy,dlespiau/clearcontainers-proxy,dlespiau/proxy