DOC: update docs for .astype with dict

[jreback]

in #13375 added dict argument to .astype

need to add a doc examp:

http://pandas.pydata.org/pandas-docs/stable/basics.html#

add versionadded tag to .astype docstring (0.19.0)

[m-charlton]

I'm willing to do this. I'm assuming that the next major release is 0.20.0. If
so the files that need changing are

docs/source/basics.rst
docs/source/whatsnew/v0.20.0.txt

I intend to replace the existing section see

Convert a subset of columns to a specified type using astype()

In [318]: dft = pd.DataFrame({'a': [1,2,3], 'b': [4,5,6], 'c': [7, 8, 9]})

In [319]: dft[['a','b']] = dft[['a','b']].astype(np.uint8)

In [320]: dft
Out[320]: 
   a  b  c
0  1  4  7
1  2  5  8
2  3  6  9

In [321]: dft.dtypes
Out[321]: 
a    uint8
b    uint8
c    int64
dtype: object

With the following:

Convert a subset of columns to a specified type by passing a dictionary,
mapping the column name to the type for that column, to astype()

In [318]: dft = pd.DataFrame({'a': [1,2,3], 'b': [4,5,6], 'c': [7, 8, 9]})

In [319]: dft = dft.astype({'a':np.float64, 'c': np.uint8})

In [320]: dft
Out[320]: 
     a  b  c
0  1.0  4  7
1  2.0  5  8
2  3.0  6  9

In [321]: dft.dtypes
Out[321]: 
a    float64
b      int64
c      uint8
dtype: object

Just to confirm, this enhancement maps a column name to a desired data type for
that column. The original text mentioned a column index, If I use an index, I
get a KeyError.

Cheers

Michael.