Update README.md

Updated to reflect how to use convertEmptyValues option

Author: terencenero007

README.md

@@ -53,6 +53,7 @@ All CLI options are optional:
 --optimizeDbBeforeStartup -o  Optimizes the underlying database tables before starting up DynamoDB on your computer. You must also specify -dbPath when you use this parameter.
 --migrate                 -m  After starting DynamoDB local, create DynamoDB tables from the Serverless configuration.
 --seed                    -s  After starting and migrating dynamodb local, injects seed data into your tables. The --seed option determines which data categories to onload.
+--convertEmptyValues      -e  Set to true if you would like the document client to convert empty values (0-length strings, binary buffers, and sets) to be converted to NULL types when persisting to DynamoDB.
 ```
 
 All the above options can be added to serverless.yml to set default configuration: e.g.
@@ -65,6 +66,7 @@ custom:
       inMemory: true
       migrate: true
       seed: true
+      convertEmptyValues: true
     # Uncomment only if you already have a DynamoDB running locally
     # noStart: true
 ```