Add the |Stable| badge on README.rst

[viniarck]

Eventually, add the |Stable| badge on README.rst, once this NApp has been considered stable by our core team. NApps are usually considered stable once e2e tests are passing and when exploratory tests also don't find unexpected issues or unwanted behavior, and finally when it's tested with a production-like scalability in number of flows and general convergence. The production-like scalability is derived from AmLight's use case and the expected number of EVCs.