Add link to Cassandra comparison

README.md

@@ -5,6 +5,7 @@ Current databases supported:
 
 + InfluxDB
 + Elasticsearch ([announcement blog here](https://influxdata.com/blog/influxdb-markedly-elasticsearch-in-time-series-data-metrics-benchmark/))
++ Cassandra ([InfluxDB Tops Cassandra in Time-Series Data & Metrics Benchmark](https://www.influxdata.com/influxdb-vs-cassandra-benchmark-time-series-metrics/))
 
 ## Testing Methodology
 In an attempt to make our performance comparison both realistic and relatable, we decided to build our benchmark suite according to real-world use cases. Micro-benchmarks are useful for database engineers, but using realistic data helps us better understand how our software performs under practical workloads.