DOC: Complete R interface section
Currently, "rpy2  R interface" documentation has blank section. Added descriptions to cover required basic topics.

@sinhrks Great to work on this! (it was a little bit a stub page now)

But I am not sure of the current status of how the conversion should be preferably done. Eg see http://stackoverflow.com/questions/20630121/pandas-how-to-convert-r-dataframe-back-to-pandas, I don't know if `convert_to_r_dataframe` is still needed (with the newer rpy2)
 Thanks. I've used rpy 2.3.8 and will check 2.4.0 spec.
 If I look at the rpy2 docs, it seems this conversion machinery `pandas2ri` was already there from 2.3.3, although there were some changes in 2.4
 @jorisvandenbossche I am ok with this
 I've briefly checked `pandas2ri` of rpy2.4.0, and found current pandas conversion looks better. `pandas2ri` doesn't convert returned rpy2 `DataFrame` automatically, and may raise `ValueError` for `DatetimeIndex`. 

But it is good if we don't have to care about rpy2 instances. I think it can be achieved by simple wrapper on `pandas.rpy2.common`, and will issue a PR for this.

```
import pandas as pd
import pandas.rpy.common as com
import rpy2.robjects as ro
from rpy2.robjects import pandas2ri

pandas2ri.activate()

iris = com.load_data('iris')
ro.r.assign('iris', iris)
returned = ro.r['iris']
type(returned)
# <class 'rpy2.robjects.vectors.DataFrame'>

df = pd.DataFrame(np.random.randn(20, 5),
                  index=pd.date_range(start='2011/01/01', freq='D', periods=20))
ro.r.assign('df', df)
returned = ro.r['df']
# ValueError: Unknown numpy array type.
```
 Something else, but maybe somewhat orthogonal to this PR, I think we could also mention the IPython `rmagic` in this doc page.
 Note rpy2 2.4+ is now the blessed "proper" location to get the rmagic via:

```
%load_ext rpy2.ipython
```

The updated rmagic lives in that sub-package in rpy2.
 @jorisvandenbossche merge when happy with this
 I think it depends a bit on the outcome of  #7385?
 @sinhrks ok, deferring this to outcome of #7385 

or if you want to document the current, then you can change (or submit a new one).
 let's redirect to `rpy2` for conversions, see #9602 
