Create README.md

Fixes T675

README.md

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+# solana_exporter
+
+solana_exporter exports basic monitoring data from a Solana node.
+
+Docker images are available on [Docker Hub](https://hub.docker.com/r/certusone/solana_exporter).
+
+## Metrics
+
+- **solana_validator_root_slot** - Latest root seen by each validator.
+- **solana_validator_last_vote** - Latest vote by each validator (not necessarily on the majority fork!)
+- **solana_validator_delinquent** - Whether node considers each validator to be delinquent.
+- **solana_validator_activated_stake**  - Active stake for each validator. 
+- **solana_active_validators** - Total number of active/delinquent validators.
+
+## Environment variables
+
+| Variable          | Description             |
+|-------------------|-------------------------|
+| `SOLANA_RPC_ADDR` | Your node's RPC URL     |