Added COCKROACHDB_SSLMODE block in README

Author: viragtripathi

README.md

@@ -35,6 +35,7 @@ export COCKROACHDB_PORT="26257"
 export COCKROACHDB_USER="your-user"
 export COCKROACHDB_PASSWORD="your-password"
 export COCKROACHDB_DATABASE="your-database"
+export COCKROACHDB_SSLMODE="verify-full"
 ```
 
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