Add roadmap page (cadence-workflow#3200)

* Add roadmap page

docs/roadmap.md

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+# Cadence Roadmap
+
+The following is a high-level quarterly roadmap of the [Cadence](https://cadenceworkflow.io/) project.
+Please contact [Cadence discussion group](https://groups.google.com/d/forum/cadence-discussion) for more information.
+
+## Q3 2019
+
+* [Resource-Specific Tasklist (a.k.a session)](https://github.com/uber/cadence/blob/master/docs/design/1533-host-specific-tasklist.md)
+* [Visibility on Elastic Search](https://github.com/uber/cadence/blob/master/docs/visibility-on-elasticsearch.md)
+* Scalable tasklist
+* MySQL support
+
+## Q4 2019
+
+* [Multi-DC support for Cadence replication](https://github.com/uber/cadence/blob/master/docs/design/2290-cadence-ndc.md)
+* Workflow history archival
+* Workflow visibility archival
+* [Synchronous Request Reply](https://github.com/uber/cadence/blob/master/docs/design/2215-synchronous-request-reply.md)
+* Postgres SQL support
+
+## Q1 2020
+
+* Service availability and reliability improvements
+* [Domain level AuthN and AuthZ support](https://github.com/uber/cadence/issues/2833)
+* Graceful domain failover design
+* UI bug fixes and performance improvements
+
+## Q2 2020
+
+* Kafka deprecation for Cadence replication
+* Graceful domain failover
+* Multi-tenancy task prioritization and resource isolation
+* UI and CLI feature parity
+
+## Q3 2020 and beyond
+
+* GRPC support
+* Parent-child affinity
+* Task priority
+* Reset 2.0 (more flexibility and handle child)
+* Safe workflow code rollout