Update README.md

Author: Allegra Chen

README.md

@@ -19,10 +19,13 @@ functions:
     handler: lambdaEdge/viewerRequest.handler
     events:
       - preExistingCloudFront:
+        # ---- Mandatory Properties -----
           distributionId: xxxxxxx # CloudFront distribution ID you want to associate
           eventType: viewer-request # Choose event to trigger your Lambda function, which are `viewer-request`, `origin-request`, `origin-response` or `viewer-response`
           pathPattern: '*' # Specifying the CloudFront behavior
           includeBody: false # Whether including body or not within request
+        # ---- Optional Property -----
+          stage: dev # Specify the stage at which you want this CloudFront distribution to be updated
 
 plugins:
   - serverless-lambda-edge-pre-existing-cloudfront
@@ -41,4 +44,11 @@ lambdaEdgePreExistingCloudFront:
   validStages:
     - staging
     - production
-```
+```
+
+### How `validStages` and `stage` properties work
+This plugin will first check for `validStages` property defined in the `custom` section. If `validStages` is used, then all the `preExistingCloudFront` events are only possible to be updated at the `validStages`. If not used, all the `preExistingCloudFront` events are possible to be updated at any stage.
+
+Then at all valid stages, the plugin checks - for each `preExistingCloudFront` event - if the provider's stage is the same as the `stage` property defined for each `preExistingCloudFront` event. If they match, then that particular `preExistingCloudFront` event will be updated.
+
+If `stage` is not used for a `preExistingCloudFront` event, then that event will be updated at all `validStages` or all stages if `validStages` is not used.