docs: Include mexmessageperbatch in the readme (#63)

* update

* update

Author: jiangmichaellll

.readme-partials.yaml

@@ -47,12 +47,13 @@ custom_content: |
 
   The connector supports a number of options to configure the read:
 
-    | Option | Type | Required | Meaning |
-    | ------ | ---- | -------- | ------- |
-    | pubsublite.subscription | String | Y | Full subscription path that the connector will read from. |
-    | pubsublite.flowcontrol.byteoutstandingperpartition | Long | N | Max number of bytes per partition that will be cached in workers before Spark processes the messages. Default to 50000000 bytes. |
-    | pubsublite.flowcontrol.messageoutstandingperpartition | Long | N | Max number of messages per partition that will be cached in workers before Spark processes the messages. Default to Long.MAX_VALUE. |
-    | gcp.credentials.key | String | N | Service account JSON in base64. Default to [Application Default Credentials](https://cloud.google.com/docs/authentication/production#automatically). |
+    | Option | Type | Required | Default Value | Meaning |
+    | ------ | ---- | -------- | ------------- | ------- |
+    | pubsublite.subscription | String | Y | | Full subscription path that the connector will read from. |
+    | pubsublite.flowcontrol.byteoutstandingperpartition | Long | N | 50_000_000 | Max number of bytes per partition that will be cached in workers before Spark processes the messages. |
+    | pubsublite.flowcontrol.messageoutstandingperpartition | Long | N | Long.MAX | Max number of messages per partition that will be cached in workers before Spark processes the messages. |
+    | pubsublite.flowcontrol.maxmessagesperbatch | Long | N | Long.MAX | Max number of messages in micro batch. |
+    | gcp.credentials.key | String | N | [Application Default Credentials](https://cloud.google.com/docs/authentication/production#automatically) | Service account JSON in base64. |
 
   ### Data Schema