Add information on requirements for osm example

The package castra is not mentioned in the README.md.

I tried conda install -c https://conda.anaconda.org/calex castra (the only copy of castra for py34 on osx-64 on anaconda) but I got the error:

    TypeError                                 Traceback (most recent call last)
----> 1 df = dd.from_castra('data/osm.castra')
      2 df.tail()

/Users/caged/miniconda3/envs/datashader/lib/python3.4/site-packages/dask/dataframe/io.py in from_castra(x, columns)
    567     from castra import Castra
    568     if not isinstance(x, Castra):
--> 569         x = Castra(x, readonly=True)
    570     return x.to_dask(columns)
    571 

TypeError: __init__() got an unexpected keyword argument 'readonly'

[jbednar]

The discussion of the Census castra files above is out of date; the Census data is now provided as census.h5 with the download scripts described in the examples directory. You can convert that to Castra or whatever you like once loaded.

[jbednar]

The OSM data is also now available, or at least a 1-billion point subset of it, so this issue can be closed.