Prepare 3.8.0 changelog (#1329)

Co-authored-by: skarimo <[email protected]>

Author: jirikuncar

CHANGELOG.md

@@ -1,3 +1,22 @@
+## 3.8.0 (January 18, 2022)
+
+IMPROVEMENTS:
+
+-   `datadog_dashboard`: Implement support for sunburst widget ([#1324](https://github.com/DataDog/terraform-provider-datadog/pull/1324))
+-   `datadog_monitor`: Add support `ci-pipelines alert` monitor type ([#1315](https://github.com/DataDog/terraform-provider-datadog/pull/1315))
+-   `datadog_security_monitoring_rules`: Raise the case limit in security rules ([#1313](https://github.com/DataDog/terraform-provider-datadog/pull/1313))
+-   `datadog_service_level_objective`: Fix SLO-correction examples ([#1307](https://github.com/DataDog/terraform-provider-datadog/pull/1307))
+-   `datadog_slo_correction`: Update documentation to list supported correction rules ([#1308](https://github.com/DataDog/terraform-provider-datadog/pull/1308))
+-   `datadog_synthetics_test`: Add GET call after create to ensure resource is created successfully ([#1312](https://github.com/DataDog/terraform-provider-datadog/pull/1312))
+-   `datadog_synthetics_test`: Add retry options to Synthetics multi step ([#1317](https://github.com/DataDog/terraform-provider-datadog/pull/1317))
+-   `datadog_synthetics_test`: Add support for websocket synthetics tests ([#1287](https://github.com/DataDog/terraform-provider-datadog/pull/1287))
+-   `datadog_synthetics_test`: Allow variables in `moreThan` operator with JSONPath ([#1322](https://github.com/DataDog/terraform-provider-datadog/pull/1322))
+
+NOTES:
+
+-   `datadog_application_key`: Deprecate `agent_rule` field ([#1318](https://github.com/DataDog/terraform-provider-datadog/pull/1318))
+-   Update Datadog client to [v1.8.0](https://github.com/DataDog/datadog-api-client-go/releases/tag/v1.8.0)
+
 ## 3.7.0 (December 15, 2021)
 
 IMPROVEMENTS: