doc: Add a better introduction sentence to README
doc: Add a better introduction sentence to README

Since the proxy is now standalone, we can do a better job introducing
the context where it's useful.

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

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