fix(cx-api): adding missing readme (#33867)

### Issue # 

Closes #33866


### Reason for this change

#33729 missing readme update for cx-api


### Description of changes

added readme

https://github.com/aws/aws-cdk/blob/eae1d4fcc500ed39973a2d796c161171894eeb41/packages/aws-cdk-lib/cx-api/README.md?plain=1#L662-L681


### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: IkeNefcy

packages/aws-cdk-lib/cx-api/README.md

@@ -658,3 +658,24 @@ _cdk.json_
   }
 }
 ```
+
+* `@aws-cdk/aws-events:requireEventBusPolicySid`
+
+When this flag is enabled:
+- Resource policies will be created with Statement IDs for service principals
+- The operation will succeed as expected
+
+When this flag is disabled:
+- A warning will be emitted
+- The grant operation will be dropped
+- No permissions will be added
+
+_cdk.json_
+
+```json
+{
+  "context": {
+    "@aws-cdk/aws-events:requireEventBusPolicySid": true
+  }
+}
+```

packages/aws-cdk-lib/cx-api/lib/features.ts

@@ -1431,6 +1431,7 @@ export const FLAGS: Record<string, FlagInfo> = {
     compatibilityWithOldBehaviorMd: 'Disable the feature flag to add the root account principal back',
   },
 
+  //////////////////////////////////////////////////////////////////////
   [EVENTBUS_POLICY_SID_REQUIRED]: {
     type: FlagType.BugFix,
     summary: 'When enabled, grantPutEventsTo() will use resource policies with Statement IDs for service principals.',