Add "maintenance mode" note to README (#606)

Only bug/security/dependency changes will be accepted.

Author: Paulo Janotti

README.md

@@ -1,5 +1,9 @@
 # OpenCensus Service
 
+*IMPORTANT:* This project is now in __maintenance mode__: only bug/security/dependency PRs will be accepted and published.
+
+Development of new features and improvements will be done on its successor project: [__OpenTelemetry Service__](https://github.com/open-telemetry/opentelemetry-service/). 
+
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