minor symfony#22417 Add a note about the ServiceLocator class in the changelog (chalasr)

This PR was merged into the 3.3-dev branch.

Discussion
----------

Add a note about the ServiceLocator class in the changelog

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

In addition to have `container.service_locator` and `container.service_subscriber` based on, this class is useful on its own as it is the one avoiding the need for having the `Container` class filling this role.

Commits
-------

92994f3 Add a note about the ServiceLocator class in the changelog

Author: nicolas-grekas

src/Symfony/Component/DependencyInjection/CHANGELOG.md

@@ -10,6 +10,7 @@ CHANGELOG
  * added "container.service_locator" tag for defining service-locator services
  * added anonymous services support in YAML configuration files using the `!service` tag.
  * added "TypedReference" and "ServiceClosureArgument" for creating service-locator services
+ * added `ServiceLocator` - a PSR-11 container holding a set of services to be lazily loaded
  * added "instanceof" section for local interface-defined configs
  * added prototype services for PSR4-based discovery and registration
  * added `ContainerBuilder::getReflectionClass()` for retrieving and tracking reflection class info