'Adds a source identifier to an existing RDS event notification
subscription.
:type subscription_name: string
:param subscription_name: The name of the RDS event notification
subscription you want to add a source identifier to.
:type source_identifier: string
:param source_identifier:
The identifier of the event source to be added. An identifier must
begin with a letter and must contain only ASCII letters, digits,
and hyphens; it cannot end with a hyphen or contain two consecutive
hyphens.
Constraints:
+ If the source type is a DB instance, then a `DBInstanceIdentifier`
must be supplied.
+ If the source type is a DB security group, a `DBSecurityGroupName`
must be supplied.
+ If the source type is a DB parameter group, a `DBParameterGroupName`
must be supplied.
+ If the source type is a DB snapshot, a `DBSnapshotIdentifier` must be
supplied.'

def add_source_identifier_to_subscription(self, subscription_name, source_identifier):

    params = {'SubscriptionName': subscription_name, 'SourceIdentifier': source_identifier}
    return self._make_request(action='AddSourceIdentifierToSubscription', verb='POST', path='/', params=params)
