add coverage badges (opensearch-project#281)

Signed-off-by: Yaliang Wu <ylwu@amazon.com>

.github/workflows/CI-workflow.yml

@@ -84,5 +84,5 @@ jobs:
       - name: Upload Coverage Report
         uses: codecov/codecov-action@v1
         with:
-          file: ./build/reports/jacoco/test/jacocoTestReport.xml
-          flags: plugin
+          flags: ml-commons
+          token: ${{ secrets.CODECOV_TOKEN }}

README.md

@@ -1,3 +1,9 @@
+[![Test Workflow](https://github.com/opensearch-project/ml-commons/workflows/Build%20and%20Test%20ml-commons/badge.svg)](https://github.com/opensearch-project/ml-commons/actions)
+[![codecov](https://codecov.io/gh/opensearch-project/ml-commons/branch/main/graph/badge.svg)](https://codecov.io/gh/opensearch-project/ml-commons)
+[![Documentation](https://img.shields.io/badge/api-reference-blue.svg)](https://opensearch.org/docs/latest/ml-commons-plugin/api/)
+[![Chat](https://img.shields.io/badge/chat-on%20forums-blue)](https://discuss.opendistrocommunity.dev/c/alerting/)
+![PRs welcome!](https://img.shields.io/badge/PRs-welcome!-success)
+
 <img src="https://opensearch.org/assets/brand/SVG/Logo/opensearch_logo_default.svg" height="64px"/>
 
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