Update Projects API documentation to include statistics parameter

* Only supported for the GET /projects endpoint

doc/api/projects.md

@@ -40,6 +40,7 @@ Parameters:
 | `owned` | boolean | no | Limit by projects owned by the current user |
 | `membership` | boolean | no | Limit by projects that the current user is a member of |
 | `starred` | boolean | no | Limit by projects starred by the current user |
+| `statistics` | boolean | no | Include project statistics |
 ```json
 [
@@ -91,7 +92,14 @@ Parameters:
     "shared_with_groups": [],
     "only_allow_merge_if_pipeline_succeeds": false,
     "only_allow_merge_if_all_discussions_are_resolved": false,
-    "request_access_enabled": false
+    "request_access_enabled": false,
+    "statistics": {
+      "commit_count": 37,
+      "storage_size": 1038090,
+      "repository_size": 1038090,
+      "lfs_objects_size": 0,
+      "job_artifacts_size": 0
+    }
   },
   {
     "id": 6,
@@ -151,7 +159,14 @@ Parameters:
     "shared_with_groups": [],
     "only_allow_merge_if_pipeline_succeeds": false,
     "only_allow_merge_if_all_discussions_are_resolved": false,
-    "request_access_enabled": false
+    "request_access_enabled": false,
+    "statistics": {
+      "commit_count": 12,
+      "storage_size": 2066080,
+      "repository_size": 2066080,
+      "lfs_objects_size": 0,
+      "job_artifacts_size": 0
+    }
   }
 ]
 ```