Add Ability For Users to Self-Assign Issues

[sicoyle]

It would be nice if a user could self assign issues to themselves and have parity with other dapr repos where they may self assign issues.

Steps
That entails having a dapr_bot.js file, like this one from dapr/dapr but simplified with only the ability to run /assign. See here for example on how to have the /assign command. This would entail needing the appropriate dapr-bot.yml file, but with the appropriate changes based on it being a different repository with different maintainers. Please ask in discord or on the PR to find out who the appropriate maintainers are.

Also, please update the the daprbot documentation in dapr/docs noting that this repo now has the /assign command available.

As a final step, please link to the daprbot documentation inside the contributing.md file so that users know what commands they can run in this repo.

inspiration for simple implementation can be found here