Update SECURITY.md with dependabot information (signalfx#187)

Based on https://github.com/signalfx/splunk-otel-collector/blob/main/docs/security.md

SECURITY.md

@@ -5,3 +5,21 @@
 Please *DO NOT* report security vulnerabilities with public GitHub issue
 reports. Please [report security issues here](
 https://www.splunk.com/en_us/product-security/report.html).
+
+## Dependencies
+
+This project relies on a variety of external dependencies.
+These dependencies are monitored by
+[Dependabot](https://docs.github.com/en/code-security/supply-chain-security/configuring-dependabot-security-updates).
+Dependencies are [checked
+daily](https://github.com/signalfx/splunk-otel-java/blob/main/.github/dependabot.yml)
+and associated pull requests are opened automatically. Upgrading to the [latest
+release](https://github.com/signalfx/splunk-otel-java/releases)
+is recommended to ensure you have the latest security updates. If a security
+vulnerability is detected for a dependency of this project then either:
+
+- You are running an older release
+- A new release with the updates has not been cut yet
+- The updated dependency has not been merged likely due to some breaking change
+  (in this case, we will actively work to resolve the issue)
+- The dependency has not released an updated version with the patch