DDB Enhanced README update - add AutoGeneratedTimestampAttribute and … (#3475)

* DDB Enhanced README update - add AutoGeneratedTimestampAttribute and other examples.

* Fix typo.

Author: debora-ito

services-custom/dynamodb-enhanced/README.md

@@ -87,6 +87,10 @@ values used are also completely arbitrary.
    // Maps a physical table with the name 'customers_20190205' to the schema
    DynamoDbTable<Customer> customerTable = enhancedClient.table("customers_20190205", CUSTOMER_TABLE_SCHEMA);
    ```
+
+The name passed to the `table()` method above must match the name of a DynamoDB table if it already exists. 
+The DynamoDbTable object, customerTable, can now be used to perform the basic operations on the `customers_20190205` table. 
+If the table does not already exist, the name will be used as the DynamoDB table name on a subsequent `createTable()` method.
 
 ### Common primitive operations
 These all strongly map to the primitive DynamoDB operations they are
@@ -363,6 +367,32 @@ Or using a StaticTableSchema with custom values:
                                        .tags(atomicCounter(10L, 5L))                         
 ```
 
+### AutoGeneratedTimestampRecordExtension
+
+This extension enables selected attributes to be automatically updated with a current timestamp every time the item 
+is successfully written to the database. One requirement is the attribute must be of `Instant` type.
+
+This extension is not loaded by default, you need to specify it as custom extension while creating the enhanced 
+client. 
+
+To tell the extension which attribute will be updated with the current timestamp, tag the Instant attribute in
+the TableSchema:
+```java
+    @DynamoDbAutoGeneratedTimestampAttribute
+    public Instant getLastUpdate() {...}
+    public void setLastUpdate(Instant lastUpdate) {...}
+```
+
+If using a StaticTableSchema:
+```java
+     .addAttribute(Instant.class, a -> a.name("lastUpdate")
+                                        .getter(Customer::getLastUpdate)
+                                        .setter(Customer::setLastUpdate)
+                                        // Applying the 'autoGeneratedTimestamp' tag to the attribute
+                                        .tags(autoGeneratedTimestampAttribute())
+```
+
+
 ## Advanced table schema features
 ### Explicitly include/exclude attributes in DDB mapping
 #### Excluding attributes
@@ -391,6 +421,10 @@ By default, the table schema provides converters for all primitive and many comm
 through a default implementation of the AttributeConverterProvider interface. This behavior
 can be changed both at the attribute converter provider level as well as for a single attribute.
 
+You can find a list of the available converters in the 
+[AttributeConverter](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/enhanced/dynamodb/AttributeConverter.html) 
+interface Javadoc.
+
 #### Provide custom attribute converter providers
 You can provide a single AttributeConverterProvider or a chain of ordered AttributeConverterProviders
 through the @DynamoDbBean 'converterProviders' annotation. Any custom AttributeConverterProvider must extend the AttributeConverterProvider