DOC Adds newline to dataframe melt
Adds newline before versionadded in `dataframe.melt` so that sphinx/rst can pick it up and render it correctly.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
Hello @thomasjpfan! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-02-05 18:22:53 UTC Great thanks @thomasjpfan 