text: Title: [BUG] The README code does not immediately work
  Body: **Minimal Code To Reproduce**
  
  ```python
  from typing import Iterable, Dict, Any, List
  
  # Creating sample data
  data = [
      ["A", "2020-01-01", 10],
      ["A", "2020-01-02", None],
      ["A", "2020-01-03", 30],
      ["B", "2020-01-01", 20],
      ["B", "2020-01-02", None],
      ["B", "2020-01-03", 40]
  ]
  schema = "id:str,date:date,value:int"
  
  # schema: *, filled:int
  def fillna(df:Iterable[Dict[str,Any]],value:int=0) -> Iterable[Dict[str,Any]]:
      for row in df:
          for col in cols:
              row["filled"] = (row["value"] or value)
          yield row
  
  with FugueWorkflow() as dag:
      df1 = dag.df(data, schema).transform(fillna)
      df1.show()
  ```
  
  **Describe the bug**
  1. FugueWorkflow needs to be imported
  2. Int will have problems on Dask because it can't hold None. Change the type to double
  
  **Expected behavior**
  It should work out of the box. All other code in the README should work as well so just run the code and make sure everything works.
  
  **Environment (please complete the following information):**
   - Backend: All engines
   - Backend version: 0.4.9
   - Python version: 3.7
   - OS: linux/windows: Both
  