Added an example for infer_freq
Added an example for infer_freq function. The initial index is built using a fixed number of periods to avoid encoding frequency in this index explicitly.

Hello @Remit! 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 2021-03-30 18:15:00 UTC This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Also this PR should target the master branch instead let's close this in a couple of days if no response @Remit I changed the target branch to master, but it will require you to rebase with master > @Remit I changed the target branch to master, but it will require you to rebase with master

@mroeschke  thanks! I did the rebase, but it seems like some checks fail for the current build. Does not look to be connected with my commit thanks @Remit 

the failures on default are known > thanks @Remit
> 
> the failures on default are known

thanks Jeff! Got it.