update readme

Author: vankop

README.md

@@ -87,6 +87,7 @@ myResolver.resolve({}, lookupStartPath, request, resolveContext, (
 | enforceExtension | false                       | Enforce that a extension from extensions must be used                                                                                                     |
 | exportsFields    | ["exports"]                 | A list of exports fields in description files                                                                                                             |
 | extensions       | [".js", ".json", ".node"]   | A list of extensions which should be tried for files                                                                                                      |
+| fallback         | []                          | Same as `alias`, but only used if default resolving  fails                                                                                                |                                                                                                                                                                                         
 | fileSystem       |                             | The file system which should be used                                                                                                                      |
 | fullySpecified   | false                       | Request passed to resolve is already fully specified and extensions or main files are not resolved for it (they are still resolved for internal requests) |
 | mainFields       | ["main"]                    | A list of main fields in description files                                                                                                                |