DOC: explain EWM 
- [ ] closes #34867
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @KrishnaSai2020! 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 2020-06-20 20:06:07 UTC @charlesdong1991 believe this is done please have a look when you can. thanks @KrishnaSai2020  @jreback np and @charlesdong1991 thanks for your help!