Create README.md (#5)

README.md

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+### Purpose of this module
+
+opendb module provides observability and configurability for rocksdb databases.
+
+### Observability
+
+When you open RocksDB with provided API:
+- opendb registers prometheus metrics
+- opendb launches a `reportMetrics` goroutine which every `rocksdb.report-metrics-interval-secs` seconds reports metrics to prometheus, `reportMetrics` goroutine performs following steps:
+  - uses grocksdb API to gather rocksdb properties/stats in text format
+    - more info can be found here: https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide#rocksdb-statistics
+  - parses rocksdb properties/stats from text and reports them as prometheus metrics
+
+List of reported metrics and their documentation can be found in:
+- source code: `registerMetrics()` function in `metrics.go`
+- corresponding grafana dashboard
+
+### Configurability
+
+opendb allows you to override default rocksdb settings, more info can be found here:
+- https://github.com/facebook/rocksdb/wiki/Setup-Options-and-Basic-Tuning
+- https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide
+
+There are 2 main scenarios:
+- create a new rocksdb database
+- open already existing database
+
+#### Create a new rocksdb database
+
+- opendb starts with default cometbft-db rocksdb configuration, see for the details: https://github.com/Kava-Labs/cometbft-db/blob/main/rocksdb.go
+- opendb overrides options which explicitly specified in appOpts (app.toml)
+
+#### Open an already existing database
+
+- opendb loads stored rocksdb configuration and starts with it
+- opendb overrides options which explicitly specified in appOpts (app.toml)