System.FinalException: Record is read-only

Because you cannot update the same record in After Insert context.

Use Before context variables in updating the same records

No need of DML in Before Insert 

trigger TestHelloWorld on Account (Before insert) 
{    
System.debug('hello World After Insert of Account !!!! ');    
for(Account a : Trigger.New)
  {        
    a.Description = 'Created from Trigger';    
  } 
}