added more details on the line chart of the rw-benchmark

Signed-off-by: Benjamin Wang <wachao@vmware.com>

tools/rw-benchmark/README.md

@@ -25,3 +25,7 @@ To generate a HTML page (read and write) based on the benchmark result csv file:
 # benchmark results from two data csv files respectively.
 ./rw-benchmark -l "main:dev" ${FIRST_CSV_FILE} ${SECOND_CSV_FILE}
 ```
+
+The read QPS is displayed as <span style="color:blue">blue</span>, and write QPS is displayed as <span style="color:red">red</span>. 
+The data in the second CSV file is rendered as dashed line if present. See example in [example/rw_benchmark.html](example/rw_benchmark.html).
+Note each line in the line chart can be hidden or displayed by clicking on the related legend.