DOC: add versionadded specifications to docstrings
We already use the `.. versionadded::` directive in the tutorial docs to indicate that a certain feature is only present starting from a specific pandas version.
But I propose that we also start to do this in the docstrings, when adding a new function/method or adding a new keyword to an existing function/method:
- When developing a package depending on pandas, you typically are using latest pandas but want to support also certain older versions. When using a certain method/function/keyword functionality, the easiest place to see if this is available is the docstring.
- Or, you also typically look at the stable docs even when using a maybe older pandas version and see functions/keywords that don't yet exist for your version.
- This is also how it is done in numpy:
  - example of new method: http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.nanmean.html
  - example of new keyword: http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.percentile.html

So if we agree on this, some todo's:
- [ ] add guidelines to the developer docs on how to add this (and start asking for this when reviewing PRs) 
- [ ] - optional - add this retrospectively to eg the new features of 0.16.0/1 (#10339 for 0.16.0/0.16.1)
