Add a "stale" bot to this repo

[vielmetti]

Add the probot "stale" bot to the repo to reduce the number of outstanding issues that are overtaken by events.

[vielmetti]

Details:

• https://github.com/probot/probot
• https://probot.github.io/
• https://probot.github.io/apps/stale/

I have this running on the "worksonarm/cluster" repo too.

[vielmetti]

Bot installed, should have results within the hour.