Update index.md

docs/en/02-intro/index.md

@@ -110,4 +110,6 @@ As a high-performance, scalable and SQL supported time-series database, TDengine
 - [TDengine vs InfluxDB、OpenTSDB、Cassandra、MySQL、ClickHouse](https://www.tdengine.com/downloads/TDengine_Testing_Report_en.pdf)
 - [TDengine vs OpenTSDB](https://tdengine.com/2019/09/12/710.html)
 - [TDengine vs Cassandra](https://tdengine.com/2019/09/12/708.html)
-- [TDengine vs InfluxDB](https://tdengine.com/2019/09/12/706.html)
\ No newline at end of file
+- [TDengine vs InfluxDB](https://tdengine.com/2019/09/12/706.html)
+
+If you want to learn some basics about time-series databases, please check [here](https://tdengine.com/tsdb).