ENH: Adds round_decimals=None to Series.value_counts() to use with normalize=True
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

When using `value_counts` with `normalize=True` with a **Series,** the result contains the relative frequencies of the unique values, and therefore sums up to 1.0. If then you need to round this frequencies to some decimals, it is not a so trivial task [[1]](https://stackoverflow.com/questions/13483430/how-to-make-rounded-percentages-add-up-to-100)[[2]](https://stackoverflow.com/questions/5227215/how-to-deal-with-the-sum-of-rounded-percentage-not-being-100)[[3]](https://revs.runtime-revolution.com/getting-100-with-rounded-percentages-273ffa70252b). So when this new **int** `round_decimals` argument is **not None**, the method applies the [Largest_remainder_method](https://en.wikipedia.org/wiki/Largest_remainder_method) to round the values to the number of decimals selected, but also to sum up to 1.0 in total.