Create a 'Y' alias for date_range yearly frequency 
 - [x] closes #9313
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [ ] whatsnew entry
 - [ ] Add alias 'Y' to docs. 

What's the best place to introduce this in docs? 

Moreover, note the inference still infers a year to 'A'. 