Updated readme

Author: Sudhakar Reddy

README.md

@@ -171,6 +171,8 @@ The rest of these Environment Variables can be set to match AWS Lambda's environ
 * `AWS_LAMBDA_FUNCTION_VERSION`
 * `AWS_LAMBDA_FUNCTION_NAME`
 * `AWS_LAMBDA_FUNCTION_MEMORY_SIZE`
+* `AWS_LAMBDA_RIE_DISABLE_EXTENSIONS` - Disables AWS Lambda extensions when working with SAM; setting this environment variable to any value other than `FALSE` will disable extensions.
+
 
 ## Level of support