Update README.md

Add a link to DataDog framework docs.

Author: Daniel Mikusa

README.md

@@ -85,6 +85,7 @@ The buildpack supports extension through the use of Git repository forking. The
   * [Container Customizer](docs/framework-container_customizer.md) ([Configuration](docs/framework-container_customizer.md#configuration))
   * [Container Security Provider](docs/framework-container_security_provider.md) ([Configuration](docs/framework-container_security_provider.md#configuration))
   * [Contrast Security Agent](docs/framework-contrast_security_agent.md) ([Configuration](docs/framework-contrast_security_agent.md#configuration))
+  * [DataDog](docs/framework-datadog_javaagent.md) ([Configuration](docs/framework-datadog_javaagent.md#configuration)
   * [Debug](docs/framework-debug.md) ([Configuration](docs/framework-debug.md#configuration))
   * [Elastic APM Agent](docs/framework-elastic_apm_agent.md) ([Configuration](docs/framework-elastic_apm_agent.md#configuration))
   * [Dynatrace SaaS/Managed OneAgent](docs/framework-dynatrace_one_agent.md) ([Configuration](docs/framework-dynatrace_one_agent.md#configuration))