changelog: defaultRateLimit entry is for 3.39 (#34215)

Test Plan: n/a

CHANGELOG.md

@@ -44,6 +44,7 @@ All notable changes to Sourcegraph are documented in this file.
 - Support account lockout after consecutive failed sign-in attempts for builtin authentication provider (i.e. username and password), new config options are added to the site configuration under `"auth.lockout"` to customize the threshold, length of lockout and consecutive periods. [#33999](https://github.com/sourcegraph/sourcegraph/pull/33999)
 - pgsql-exporter for Code Insights has been added to docker-compose and Kubernetes deployments to gather database-level metrics. [#780](https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/780), [#4111](https://github.com/sourcegraph/deploy-sourcegraph/pull/4111)
 - `repo:dependencies(...)` predicate can now search through the [Go dependencies of your repositories](https://docs.sourcegraph.com/code_search/how-to/dependencies_search). [#32658](https://github.com/sourcegraph/sourcegraph/issues/32658)
+- Added a site config value `defaultRateLimit` to optionally configure a global default rate limit for external services.
 
 ### Changed