add roadmap (zeromicro#764)

ROADMAP.md

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+# go-zero Roadmap
+
+This document defines a high level roadmap for go-zero development and upcoming releases.
+Community and contributor involvement is vital for successfully implementing all desired items for each release.
+We hope that the items listed below will inspire further engagement from the community to keep go-zero progressing and shipping exciting and valuable features.
+
+## 2021 Q2
+- Support TLS in redis connections
+- Support service discovery through K8S watch api
+- Log full sql statements for easier sql problem solving
+
+## 2021 Q3
+- Support `goctl mock` command to start a mocking server with given `.api` file
+- Adapt builtin tracing mechanism to opentracing solutions
+- Support `goctl model pg` to support PostgreSQL code generation
+
+## 2021 Q4
+- Support `goctl doctor` command to report potential issues for given service
+- Support `context` in redis related methods for timeout and tracing
+- Support `context` in sql related methods for timeout and tracing
+- Support `context` in mongodb related methods for timeout and tracing