[docs] add draft roadmap

A high level description of what's coming next from both the network and the features/code perspective.

ROADMAP.md

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+# Roadmap
+
+Currently, builders can:
+* Write and test Move smart contracts,
+* Spin up a local Sui network,
+* Publish and run Move smart contracts on a local network.
+
+In the coming months, we will release:
+1. A public devnet that allows Sui devs to do all of the above on a shared network powered by Mysten-operated authorities
+2. A public testnet that onboards a diverse set of non-Mysten authorities to the network
+3. A public mainnet with real assets and production applications!
+
+A more fine-grained description of the upcoming features and improvements to the Sui codebase follows.
+
+# Ongoing Work and Upcoming Features
+
+### Internal Devnet
+* Constantly running Sui network
+* Productionizing network stack
+* Benchmarking throughput and latency in various configurations
+
+### Protocol Stabilization
+
+* Implementing reconfiguration and staking
+* Ledger and state checkpoints
+* Aligning Gateway Service Rust/REST/wallet API’s
+* Finalizing REST data model (aka SuiJSON)
+* Finalizing core data types
+* Authority "Follower" API's to support replicas
+* Integration of shared objects and consensus
+* Selecting principled gas costs
+* Event indexing hints
+
+### SDK and Ecosystem
+* Block explorer
+* Key management and wallet prototypes
+* Support package publishing via REST API
+* More informative error messages
+* More convenient API's that hide gas object selection and nested object authentication
+
+### Move Development Improvments
+* Allow objects used in authentication, but not passed to entrypoints
+* Explicit syntax and compiler enforcement for entrypoints
+* Adding Move Prover specs to the Sui framework and verifying in CI