Added fallback store feature

docs/options.md

@@ -14,3 +14,5 @@ The `ObjectRetriever` constructor accepts an associative array of options. All o
 | `logger` | Logger implementing `Psr\Log\LoggerInterface`, which receives info messages about object retrieval. | `null` |
 | `timeout` | Total timeout for API requests, in seconds. | `20` |
 | `connect_timeout` | Timeout for establishing a connection to the API, in seconds. | `5` |
+| `fallback_store_directory` | Writeable directory in which copies of retrieved objects are saved and returned when the API fails with a 5xx error, times out, or the request fails without a response. Requires `fallback_store_scope`. See [caching](caching.md#fallback-store). | `null` |
+| `fallback_store_scope` | Which objects are saved in the fallback store: `'*'` for all objects, or a list of namespaces (domains without `coid://` prefix) as a comma-separated string or an array of strings. | `null` |