config: add rocksdb as a db backend option (#4239)

Closes #3831

The support for rocksdb was added a while back in
https://github.com/tendermint/tm-db/pull/12. This commit merely updates
the documentation.

docs/tendermint-core/configuration.md

@@ -39,7 +39,7 @@ moniker = "anonymous"
 # and verifying their commits
 fast_sync = true
 
-# Database backend: goleveldb | cleveldb | boltdb
+# Database backend: goleveldb | cleveldb | boltdb | rocksdb
 # * goleveldb (github.com/syndtr/goleveldb - most popular implementation)
 #   - pure go
 #   - stable
@@ -51,6 +51,10 @@ fast_sync = true
 #   - EXPERIMENTAL
 #   - may be faster is some use-cases (random reads - indexer)
 #   - use boltdb build tag (go build -tags boltdb)
+# * rocksdb (uses github.com/tecbot/gorocksdb)
+#   - EXPERIMENTAL
+#   - requires gcc
+#   - use rocksdb build tag (go build -tags rocksdb)
 db_backend = "goleveldb"
 
 # Database directory