cfg: make status_rules optional

Author: yasunariw

documentation/config_docs.md

@@ -32,7 +32,7 @@ Refer [here](https://docs.github.com/en/free-pro-team@latest/developers/webhooks
 | `main_branch_name` | main branch used for the repo; filtering notifications about merges of main into other branches, and constraining prefix rule application | main branch related features disabled |
 | `label_rules` | label rules config object | required field |
 | `prefix_rules` | prefix rules config object | required field |
-| `status_rules` | status rules config object | required field |
+| `status_rules` | status rules config object | all status notifications are ignored |
 
 ## Label Options
 

lib/config.atd

@@ -27,7 +27,7 @@ type label_rules = {
 type config = {
   prefix_rules : prefix_rules;
   label_rules : label_rules;
-  status_rules : status_rules;
+  ~status_rules <ocaml default="{allowed_pipelines = Some []; rules = []}"> : status_rules;
   ?main_branch_name : string nullable; (* the name of the main branch; used to filter out notifications about merges of main branch into other branches *)
 }