feat: add contributor over time graph to README (#5531)

README.md

@@ -93,6 +93,10 @@ You are very welcome to communicate with the developers and users of Dolphin Sch
 1. Join the Slack channel by [this invitation link](https://join.slack.com/t/asf-dolphinscheduler/shared_invite/zt-omtdhuio-_JISsxYhiVsltmC5h38yfw).
 2. Follow the [Twitter account of Dolphin Scheduler](https://twitter.com/dolphinschedule) and get the latest news on time.
 
+### Contributor over time
+  
+[![Contributor over time](https://contributor-graph-api.apiseven.com/contributors-svg?chart=contributorOverTime&repo=apache/dolphinscheduler)](https://www.apiseven.com/en/contributor-graph?chart=contributorOverTime&repo=apache/dolphinscheduler)
+
 ## How to Contribute
 
 The community welcomes everyone to contribute, please refer to this page to find out more: [How to contribute](https://dolphinscheduler.apache.org/en-us/community/development/contribute.html).