Add documentation for packages API endpoint

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/api/README.md

@@ -65,6 +65,7 @@ The following API resources are available:
   - [Project templates](project_templates.md) (see also [Templates API Resources](#templates-api-resources))
 - [Protected branches](protected_branches.md)
 - [Protected tags](protected_tags.md)
+- [Packages](packages.md) **[PREMIUM]**
 - [Repositories](repositories.md)
 - [Repository files](repository_files.md)
 - [Repository submodules](repository_submodules.md)

doc/api/packages.md

+# Packages API
+
+## List project packages
+
+> [Introduced][ee-9259] in GitLab 11.8.
+
+Get a list of project packages. Both Maven and NPM packages are included in results.
+When accessed without authentication, only packages of public projects are returned.
+
+```
+GET /projects/:id/packages
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id`      | integer/string | yes | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user |
+
+```bash
+curl --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/projects/:id/packages
+```
+
+Example response:
+
+```json
+[
+  {
+    "id": 1,
+    "name": "com/mycompany/my-app",
+    "version": "1.0-SNAPSHOT",
+    "package_type": "maven"
+  },
+  {
+    "id": 2,
+    "name": "@foo/bar",
+    "version": "1.0.3",
+    "package_type": "npm"
+  }
+]
+```
+
+By default, `GET` request return 20 results at a time because the API results
+are paginated.
+
+Read more on [pagination](README.md#pagination).
+
+This API is for listing project packages. For how to upload or install 
+Maven or NPM packages please visit [Packages](../administration/packages.md) documentation.
+
+[ee-9259]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9259