Added dataclass into dataframe API reference
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
Added dataclass constructor for dataframe into the Pandas in pandas/init.py line 363 and 426.
thanks @taytzehao 