Add file-like object to docs for Feather
`read_feather` and `to_feather` accept file-like object. For example, something like this works:
```
# python -uc "import pandas, sys; print(pandas.read_feather(sys.stdin))" < a.feather 
    id
0  1.0
1  2.0
2  3.0
3  NaN
4  5.0
5  6.0
6  7.0
7  8.0
# python -uc "import pandas, sys; pandas.to_feather(pandas.read_feather(sys.stdin), sys.stdout)" < a.feather > b.feather
```

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
