Adding stale bot GitHub action

# Stale bot Warns and then closes issues and PRs that have had no activity for a specified amount of time.

## How it works
- After a period of inactivity, a label will be applied to mark an issue as stale, and optionally a comment will be posted to notify contributors that the Issue or Pull Request will be closed.

- If the Issue or Pull Request is updated, or anyone comments, then the stale label is removed and nothing further is done until it becomes stale again.

- If no more activity occurs, the Issue or Pull Request will be automatically closed with an optional comment.

Will be helpful to manage the workflow later on.
I want to work on this

Remember to use helpful labels and milestones. If you use the "help wanted" label, Code for America will [promote it widely](http://www.codeforamerica.org/geeks/civicissues).